it doesnt ignore defense, it takes (what ive heard) 10% hp from you, and adds to your damage. If it ignored defense i think i would be hitting for a bit more than 200+ :p
this is actually called "Soul Eater". it doesnt actually raise your attack ability like berserk or last resort, instead when you hit a mob it does a significant amount of damage. the price is every hit that you land succesfully on the mob also consumes HP from you.