please help me understand how Blood Hunt works~

I'm playing Ritualist, can make pretty good physical damage by Wirling/Twister and I can see huge dark Blood Loss life bar( much bigger than enemy's normal life), but then I hit it with a Blood Hunt, only remove a part of the life, nothing big.

But I can see like one button Blood Hunt BD over the youtube( Bloodmage, Titan), they don't make the Blood Loss that much like I did(just start to loss only a few blood), but they can just one hit to make the Boss defeated.

What happened? What did I miss? Please~