Stun

Whenever a player or monster takes damage, there is a chance they will be stunned. A stun interrupts whatever that creature was doing while a brief animation is played. The chance to stun is based off the damage done in a single attack compared to the target's maximum HP. The default length of stuns is 350ms. The duration of stuns can be altered by reduced block and stun recovery, increased stun duration, and similar modifiers.

The formula used for determining whether or not a stun occurs is: stun_chance = 200 * damage / defender_effective_max_life

Where defender_effective_max_life is what the maximum life of the creature being hit would be in a single-player map. Increases to monster life from a party of more than one player do not affect defender_effective_max_life.

For a player with Chaos Inoculation, their defender_effective_max_life is whatever their max life would be if they did not have Chaos Inoculation. Reduced stun threshold modifiers reduce the value of defender_effective_max_life. For example, 25% stun threshold reduction means you treat their maximum life as only 75% as much as it actually is, meaning you stun them easier.

If the stun chance would be less than or equal to 25%, it's ignored, so you need to deal more than 12.5% of effective maximum life to have a chance to stun.

As long as you have greater than 0 Energy Shield, you have a 50% chance to avoid stun.