When hate shifts to another target while this guy is casting a spell, that spell will be interupted no matter what, as long as the spell doesn't go off before the hate shifts. This is very important when you first pop it (have someone there ready to voke it when it starts casting a spell on the person who popped it).
Provoke alone won't interupt the spell if it doesn't generate enough enmity to pull hate off of the mobs target.
This item works at 50% mp or under, in other words under 51%.