How exactly does Piercing work?

Okay, so we got four damage types: Normal, Elemental, Piercing, and Shadow.
The general idea I get from the damage types is this:
Normal is neutral from just being some sharp blade.
Elemental is effective against Constructs because it can freeze or fry circuitry, or melt or corrode armor.
Elemental is effective against Undead because they utilize elements to cut though bone and crap. Fire burns undead easily, Cold makes bone brittle and Shock... It does something.
Shadow is effective on Slime because... Royalty is weak to evil?
Shadow is effective against Gremlins as both Shadow and Elemental are usually opposites.
Piercing is effective against Beasts and Fiends.
It's effective against just those two? Can someone explain that while leaving out game mechanics?
Also I've heard plenty of times that Piercing is basically Armor Piercing. But it has little effectivity against Constructs. Riftlocker's description says that it's Armor Piercing(somewhat).
So what I'm asking is... How is Piercing effective against Fiends and Beasts, and ineffective on Constructs?

Cautery Sword has Rock Salt in its recipe.
Now that I think about it... I think Shadow is effective against Slimes because it can melt collagen. Collagen is what Jell-O is made of, people. Search it up if it makes you feel better.

This is just my theory or what not.
I'm guessing the piercing damage type originated from the "toothpick" swords. I mean it would make sense since they're basically rapiers. These are swords that specializes in thrusting (not that type of thrusting) its pointy end to do some serious damage by piercing through the enemy's body (see what I did there). Unlike regular swords it inflicts damage by hacking through the enemy with its edge.
With this in mind it seems reasonable for these "toothpicks" to easily poke through these squishy beasts and into their vital organs. I guess this works with silkwings, devilites, I don't know about trojans. But how would it poke through metal? I guess this is why they resist piercing damage. Plus same thing with slimes, nothing really happens when you poke them, have you tried to poke Jell-O with a toothpick, nothing really happens.
- However my explanation doesn't make sense with piercing guns. Especially with the Riftlocker, look at its description:
-
"A deadly handgun said to be capable of sundering heavy armor with a single shot. These firearms proved instrumental during the siege of Isora in breaking through the war machines of the Morai."
Whaaatt?????
If the game says constructs resist piercing damage why would the description be this? Maybe the devs just wanted piercing guns since it would have been weird with just shadow and elemental guns.
Honestly I wouldn't think about this too much, after all we're in a game that gives you bonuses by wearing some kitty kat hats.

Fun fact: flourishes were not in the game at launch. Piercing damage thus can be assumed to not originate from there.
Pierce I assume is still a sort of poke thing however. All original pierce weapons relied on a sort of splinter mechanic - ie, if you stab something with it, it'll go into it.
Piercing is not effective against constructs due to constructs having metal plating. The "splinter" thus sort of... bounces off.
Piercing is not effective against slimes since slimes are more of a single cell organism. When you poke them they just sort of glorp back together with no real damage. The only way to hurt a slime with a piercing weapon is if your poke was fast enough to break the surface tension AND still be going fast enough to hit their core do-dads. Meanwhile, shadow damage just sort of melts them.
Beasts and fiends both have very light skin. This makes them highly susceptible to piercing damage. Gremlins have thick and unruly fur, and on top of that, likely thick clothing due to all of their construction and deconstruction they do. They aren't strong to pierce but aren't weak to it either.
At least, these are all the reasons I've sort of thought up for it all. Makes sense to me, but I remember someone wanting to reclassify gremlins as "weak to pierce", and that sort of makes sense, but with a trio of damage types and a sextuplet of monster families assigning 2 to each makes sense.

I am fairly certain that constructs' strength against piercing damage is that while their armor can surely be pierced through, they have no vital weak points that merely piercing could simply hit for critical damage. And piercing damage will cause nowhere as much damage on the completely metallic body as blunt strength would. Beasts and fiends generally carry no protective features on their body that would allow them to stand up to impalement or conventional gun shots, rendering their organs into easy targets to exploit.
Shadow damage can also be attributed to a variety of the following two things: malevolent and/or corruptive energy and dark matter/anti-matter. These two attributes will easily conflict between each other to some extent, as a rock salt bomb is by all means devoid of evil energy. However, Gremlins could well be weak to shadow damage due to their susceptible nature of being easily corrupted and carrying generally unstable weapons and gears.
Elemental damage is much more blurry to define when it comes to energy in general, but strong heat signatures and extreme temperature differences do play a good part in how it's dealt. The Hot Edge is a good example of this. Beasts and Gremlins' physiological adaptability in a variety of environments grants them natural resistance to such, while constructed and undead entities have an especially fragile composition in this regard.
And, of course, normal damage is very easily defined by the mass and density of the strikes dealt.

Normal vs everything- Hack, slash, stun
Piercing vs Construct- Pokey spines bounce off of metal. Plink, plink, plinkplinkplinkplink
Piercing vs Jelly- Pokey spines just stick into the jelly and don't smash it or dissolve it. Glorp.
Piercing vs Gremlin- Metal armor but soft bodies. (gremlin noise)
Piercing vs Undead- Bone splinters, cloth gets holes in it. blargh
Piercing vs Beast- just go hunt something with a spear and tell me it doesn't work.
Piercing vs Fiend- THE LIGHT OF TRUTH PIERCETH ALL WHO ART UNWORTHY. ABANDON THY WICKED WAYS BEFORE THOU BECOMEST AS FODDER FOR THE CANNON; AS PREY FOR THE ARROW OF LIGHT THAT SOARETH THROUGH THE HEAVENS AND STRIKETH TRUE ON A COAL-BLACK, DESOLATE HEART, HOLDING FAST EVEN AS ITS TARGET IS RENT ASUNDER INTO THE ASH AND BRIMSTONE OF THE ONE TRUE HELL. SEEK SALVATION QUICKLY, LEST THOU BECOMEST OVERRUN AS THE HOLY CITY TO THE BARBARIAN HORDE.
Any questions?

Fangel pretty much captured how I envisioned it working, but I don't think these weaknesses are necessarily set in stone at all or necessarily have reasons to them. During development, slimes were weak to elemental, which makes sense for them.
Would it make sense for gremlins to be weak to piercing? Sure. They're akin to beasts in many ways. What about fiends to shadow? They deal in shadow but also it corrupts them, perhaps. There's a lot of ways to go with things and we only make sense of them because that's what we do.

As far as Fiends are involved, could you corrupt them more than they are already corrupt in the first place?

It was just an example and it doesn't matter. If they put it in the game it would indeed corrupt them further. They didn't, so perhaps no but they could have and that's what matters.

Piercing - Physical force concentrated in a small target area, with the aim of penetration (Blitz Needle, Splinter Bomb)
Elemental - Manipulated natural energies like heat and electricity (Glacius, Storm Driver)
Shadow - Otherworldly destructive energies (Gran Faust, Graviton Bomb)
Piercing:
VS Beasts: Beasts don’t wear armor. Piercing attacks can penetrate their elements-resistant hide rather easily, causing internal injury. (Effective)
VS Fiends: Save for the ties worn by Pit Bosses, Fiends don’t wear armor either, and they presumably don’t even have hides like Beasts. See above (Effective)
VS Gremlins: Gremlins wear cloth armor that stops pierce attacks only to a certain extent, though it presumably provides good protection against Cradle’s elements. (Normal)
VS Undead: IDK
VS Construct: The metal plating of constructs is relatively harder to penetrate, reducing eventual damage compared to other monster families. Furthermore, internal circuitry is not damaged as much by small holes than, say, extreme heat or electrical surges (Ineffective)
VS Slimes: The heavy viscosity of Slimes’ outer layers excel at stopping physical penetrating attacks by slowing them down drastically, lowering overall injury. Also, it suffers less injury than Beasts as it lacks internal organs that would suffer from penetration (Ineffective)
As an afterthought, the reason as to Shadow attacks’ effectiveness is probably due to Slimes’ outer layers and Gremlins’ cloth armors, though adapted to Cradle, being more unequipped to deal with such otherworldly energies.

The above comments said everything there was to say about piercing besides one thing
Piercing vs Undead : normal damage
I assume undead are immune to pain being ghosts and zombies. They still get damaged but there is no vital organ to target with piercing attacks.
Also about shadow attacks
Shadow attacks are super effective against gremlin and slime type enemies because I believe shadow attacks are super effective on enemies with a certain degree of intelligence and no inherent shadow resistance ( undead & fiends )
That's the only connection between gremins and slimes.
Also some shadow weapons have descriptions where they say they make weak minded knights go crazy . Proof that weapons of dark nature mess with your head.

Piercing vs Undead : normal damage
This could be stated in a much clearer way, that doesn't involve the phrase "normal damage".

Yunno, when Fangel made his first response I was thinking of something a little different.
If Beasts and Fiends had such light, fragile hides then you could just step on everything to kill them.
I was thinking that they had thick hides that were a little resistant against slashing, and similarly to Heavy Armor they'll still suffer from the blunt force of the weapon.
Piercing would likely be the only weapon capable of piercing through the hide easily.
And Gremlins... I dunno, they probably use painkillers or chemicals or something like that to resist the power is stabbing attacks.
And Undead... You're just chipping parts off them just like any Normal slashing weapon would.

How shadow energy work, by MDJ's head canon:
Shadow energy is the opposite of mist energy, while mist energy built up most of the clockwork that you see in the game, shadow energy do present in the form of fiend, undead and the swarm.
Raw swarm energy cannot be used by any spiral tech, since it is an energy of pure chaos and destruction, anything built from the swarm will collapse almost immediately due to its construction defies real world logic. However, at a limited quantity, shadow energy can be fused with mist energy based material on a more permanent basis (ie, acheron and gran faust).
So, what to all of this have to do with its effectiveness against slimes and Gremlins?
Slimes - their body are mostly consists of homogeneous fluid, shadow energy has a tendency of nullify and repel any mist energy that comes into its vicinity, with swords like acheron, the swarm energy forms a field around the blade and as it cuts through the slime, the swarm energy react violently with the slime's single celluar body, causing it to burst and loose its surface tension. This explains why the dark energy is neutral against beast and construct since their body structure are more complex. Fiend and Undead are consist of dark energy too so the blade wont cut through as easily.
Gremlin - Gremlin was once the creation of the core, but Tinkinzar, saw the fate of the Almirian empire and knows that his kingdom is next in line to fall decided that he was going to defy the will of the core and live on his own accord. For this end, he trapped the will of cradle into the metal shell of the core, effectively taken over as the ruler of cradle. As retaliation, the will of cradle marked the entire Gremlin race for death, swarm, obeying the will of cradle, seek out and destroy Gremlin wherever possible. So in another word, we are doing the swarm's job.

The Swarm is actually the opposite of The Core, and probably the real reason why it was stated that Tinkinzar technically "betrayed The core was so he could harness the Swarm by himself; as to what Herex did. The Almirian Kingdom was "pulled in" originally to protect The Core, but before that, it was defeated and then turned into what we saw now, and also, it wouldn't disturb The Core, since the warriors and denizens are designed to protect the kingdom, and so The Core made a smart move to use shadow beings, since Gremlins are weak to that. Defeating Vanaduke allowed Herex(who happened to know what the Spiral Order was up to) to access the last area to unleash the Swarm, and harness it by himself, as what we saw when he "tried" to do so in the Royal Jelly Palace. The Spark tokens also give a hint as to what each specialized type comes form, though I'm pretty sure Piercing, Elemental, and Shadow are Light, Nature, and Darkness respectively(Heaven, Earth, and Hell?), but these types may be quite mixed, and so the Spark tokens may give a better description. Schemetics for the first weapons are made way before the Spiral Order began the journey to space, and so happened to see Cradle as another planet type similar to their home planet(even the energies), and most likely they were sent out to inhabit planets as colonies. Though there's still no explanation as to what shot the Ark down(Ark...sounds familiar...).
As for the Morai, their siege weaponry probably had the same armor type as Gremlins, hence why Riftlocker had such a description.

Someone probably already said it, but:
You can't stab robots because they're metal
Slimes are like rubber. They're too squishy to stab, they stretch with the blade and they self-heal from pokes.
Beasts are weak to piercing because stuff like bear traps and arrows and the stuff you usually would kill wild animals with.
Fiends....well beats me, actually.

Shadow: Destroys via plunging it's victims into complete and total entropy, to create clean, perfect slashes or bullet holes.
Heavily armored enemies are neutral to this, as they fairly resist it's power, while gremlins/slimes wear none, and as such get slashed apart, being organisms that rely heavily on staying whole. Meanwhile, undead and fiends, as a byproduct of the chaotic energies, take no effect from this.
Piercing: The traditional method, stabbing parts of the body to destroy internal processes.
Earth animal-like enemies such as wolvers or gremlins still have organs, and therefore get decimated by this, and as such, fiends may also apply for the same reasons. Constructs and slimes have no organs to hit within their structures, both for the haphazard construction of constructs (their circuitry could be anywhere on their body), and for the structure of slimes, which is only limited by volume.
Elemental: Overwhelming everything with natural forces such as fire and electricity.
Bone and steel are destroyed by this since they have no natural resistance in terms of elemental composure, while all other species occur semi-naturally and as such have built up resistance to this. Considering how Cradle is so chaotic in comparison to most planets, poisoned by Mist radiation, and infested with disasters, the presence of this resistance should be commonplace.

The Swarm is actually the opposite of The Core, and probably the real reason why it was stated that Tinkinzar technically "betrayed The core was so he could harness the Swarm by himself; as to what Herex did.
'betrayed' could be understood in several way, the founding fathers betrayed the British to establish united states of America. And I don't recall anything stating in the lore that Tinkinzar knows how to harness the swarm, only Herex. To quote from one of his message:
Herex no longer serves as one of the Nine.
Only Herex sees and hears the Truth. Only Herex will prevail.
Follow Herex and you will be spared.
Speaking of Vanaduke, most of his guard, the zombies, are weak against elemental damage, and elemental damage is bread and butter to Gremlins, how do you explain that? Perhaps Tinkinzar knew not to mess with the core and Herex, being a bit of a prick, decided to go against it?

Or a skull, so to speak.

Lichens and polyps do have an internal organ, the nucleus.
But when it's surrounded by a foot of squishy material with high surface tension it's hard to poke the center with an extreme amount of force to cause high damage.

Considering the physiology of a Spiral Knight, I would say about four to six feet.
And why are slimes strong against pierce? You'd expect regular slicing to be to tough against gelatin, and might make the sliced slimes to split into multiple sentient rectangles. Or at least, elemental should work more against slimes, since even the Cautery Sword mentions that heat is better for slime dispatchment.