Exact effect of Defense Up! and other buffs
Hello,
Since a couple years ago, in the arcade and in missions, monsters and treasure boxes will sometimes drop temporary buffs. There is a defense one, a damage one, and a speed one.
The effect of the speed buff is pretty obvious, and I doubt anyone cares about the exact values.
The damage buff is self explanatory. You get increased damage for a short time. It should be pretty easy to figure out the math behind it since exact damage information is available on every hit. For some reason, the wiki contains close to no information about it.
And the last one, the defense buff, seems extremely obscure to me. What does it do? Defense and resistance are different stats in this game, so I would bet it only affects shields. But what's the exact difference? does it have any impact on status effects, or just pure damage?
The caption "Defense up!" doesn't really help, I can't find any information about it anywhere, not even a wiki page that aknowledges it's existence. Also, I've been playing in french for a long time (please forgive my bad english), and the caption is even weirder. It's "Défendez-vous!", which roughly translates to "Defend yourselves!". It sounds more like a boss is gonna come to attack you instead of you having just obtained a buff, but I guess it's just a terrible translation.
I didn't really care about this information before, since I try to avoid attacks anyway, so the defense buff doesn't affect my playstyle. But I recently noticed that Drakon's second spell (fire barrier or something) also gives the same caption when activated, and the same status effect appears next to your health bar. I'm trying to figure out exactly how it behaves to use my Drakon more efficiently.
Thank you very much
What I meant by that is to ask if this specific buff had any effect on my resistance to status effect. For example, a shield that protects against fire (even though it's still pretty obscure how this works), would it protect "more" from fire during the buff, or does it only affect it's overall resistance to damage. I thought maybe the boost only affects shields because on armors and helmets, it's called resistance, not defense, so it would make sense to be unaffected by the "Defense up!" buff, but it's only a guess, I'm not sure about that.

Link
I don't specifically recall testing status resistances, but I don't believe they get the multiplier. You can test that claim, if you are skeptical.
I did not test shields. You might be able to poke the researchers in this thread to get an answer with respect to shields.
Additionally, this may be of interest as well.

Defense Orb does not affect Shields at all.
I occasionally got a defense orb in midst of fight in D26 while I tested its attacks and the outcome has been the very same amount of Pixels left to shield health.

Thanks, Skepticraven. I thought that it had been studied, but I just couldn't find it.
So the short answer is: The defense booster seems to increase the damage protection of your armor by 25%.
(The thread does not actually show the data needed to support this conclusion.)

Yup. That brief study was unfortunately prior to when I set up my recording spreadsheets.
I'm unsure of how precise the 25% value is, especially since the damage boost pickup is 20%.
This is a good question. After some Google searching, I have not found a clear answer. So it's hard to find, if it exists at all.
Presumably the defense boost increases your defense (armor damage protection). When an incoming attack hits you, your damage protection gets subtracted from the incoming damage, and whatever is left over is subtracted from your health. So probably the defense boost increases your damage protection and therefore decreases the amount of damage that you take.
Some of your other comments --- about damage vs. status, and shields vs. armor --- don't make sense to me. Why do you think that the boost affects shields? Why are you considering status? If you haven't done so already, consider reading the wiki pages Defense, Damage, Shield, and Shieldbearer's Guide.