Loosing the healthbar can be even more simpler. Make the character loose focus, waver and bleed. That way the player can see when he needs to kick back and gather his strength and when to push on just by looking at his character.
A fine example of this is Getaway.
I believe that happens when the curse of Caine manifests itself differently, resulting in a vampire with different aspects. Not sure though.
<{POST_SNAPBACK}>
Yeah, it's like that.
Doesn't happen that often though.