I just gained this ability on my Arcanist class. It states: 20% chance for an attacker to deal 10% less damage. Lasts 20 seconds on a 180sec recast timer.

In other words: if I put this on myself, or even on a tank while in a dungeon, it will reduce the incoming damage on him by a grand average of 2% while it is up. Compare this to other defensive cooldowns on 120 or 180sec recast timers, which offer 15%-20% reductions in incoming damage.

Is there something I am missing here? Does the ability do something else that the text doesn't document? Because as far as defensive bonuses go, this thing doesn't even seem worth a slot on the second hotbar in all honesty...