It will fire 20% of the time when your attack hits, for an overall 28% chance of at least one mitigation effect, and a 2% chance of both stun and knockback.
kinetic combat chance for knockdown is better than the knockback ( unless you WANT knockBACK), since you dont have to chase after the effected target.
I am an ebil markeeter and will steal your moneiz ...correction stole your moneiz. I support keeping the poor down because it is impossible to make moneiz in this game.
If I slot say, a Stupefy 20% chance for knockback (From Stun sets) to my Ice/Stone tank's Stone Fist power, which has a 10% chance to stun, does that mean it will fire 20% of the times I hit with my attack, or 20% of the times my stun is successful (so 2% of the times I hit with my attack.)?