I have a question about the Retribution skill.
http://wiki.guildwars.com/wiki/Retribution

I understand its an Enchantment Spell, also that it removed 1 bar of energy regen, I can cast it on others.
What I dont understand is this:
While you maintain this Enchantment, whenever target ally takes attack damage, this Spell deals 33% of the damage back to the source (maximum 5...17...20 damage).
Does this mean, if I cast this spell on an ally, that when they get hit, I take a portion of the damage? Or does the spell itself do some of the damage?
Thanks for help