Code:
register_event("PTakeDam", "PTakeDam_function", "a")
register_message(50, "damage_function")
PTakeDam provides the amount of damage dealt.
50 (it has no name) is called directly after PTakeDam, and provides the id's of the victim(2) and killer(3)
I'm still not certain, though, when to use register_event() and when to use register_message()
EDIT: I checked again, and I'd go with TSHealth instead of PTakeDam.