HPCNT programs trigger when a mob reaches a certain % of their max hitpoints. If they have 10k total when fully healthy, and you set it to trigger at 50%, then roughly the time they hit 5k hps, the program will begin. Syntax is: addmprog HPCNT %of total hps the program should begin at.

Example:
say Hey now, that kinda hurts a little bit! Stop that!
say C'mon $n, can't we all just get along?

Usually the mob will do this every few ticks or rounds once it gets down to the % of hitpoints specified. Please note, it will continue doing this until either A: it goes above that %, B: another program kicks in for a lower %, or C: it dies.