I always suspected it just couldn't be felt.. like, really.. adding 1 sec when heavy swing takes about 2.5 - 3 seconds..

Anyway, first second and a half is wasted by activation animation, so I guess I've never dished out more than 3 buffed heavy attacks out of this ever. It's a real pain to activate all the time, I'd much rather see it as a toggle, or at least 1 minute duration buff.. since it's no longer group buff.