What does "eye for an eye" actually do?

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...

The buff has a 20% chance to give the attacker a debuff that lowers its damage dealt by 10%. That's actually quite a huge mitigation tool, pair that with four jobs (SMN,BLM, SCH, WHM) being able to equip it, good rotations on the skill can keep damage down for a majority of any fight.

You can't cast it on yourself. The barrier itself lasts 30 seconds but each debuff that it puts on enemies when they hit the one with the shield lasts 20 seconds which means it can potentially be on them for up to 40 seconds from just one cast.
It's a party skill obviously and very useful as Tricksta said.

Oooh! So I misread the tooltip after all.

If this actually debuffs attacking enemies, that makes it way better than I thought. Thanks for clearing this up! :)