x + mouse2
Hold down the key to bring up your shield and hit the button to attack.
Shield bash would be a weak knockback attack with a *very* small chance of stun. It's the kind of attack that would be useful in a pinch, like when you're cornered. Now, there'd have to be something to stop knights from spamming this attack though, so maybe it could take down your shield a level (one level closer to breaking). Either that, or a cooldown.
I think shield bash would add a little more variety to the attacks in SK.
Shields already knockback.
If you shield and a monster is inside the shield's bubble radius, you knock them away.
It doesn't deal damage, or stun them, but it knocks them away a fair distance and it's quite useful for getting past groups of monsters.