The passive tree node for "Flurrying Flames" is supposed to give stacks of "Unleashed Fury" upon "Each time you hit an enemy", yet I'm not sure if the way it currently works is the intended way. If you have the Bladestorm modifier "Unfettered Determination", where you can hold down the key to prolong the attack, any hits after the initial cast do not grant "Unleashed Fury" stacks.