Power word spellsWhat kind of things are you thinking of, specifically, which trigger on hit points?
The Contingency spell requires the character in the game world to specify the circumstance. This necessitates that the circumstance be a thing happening within the fiction. I don't think I'd let them specify a trigger based on a certain number of hit points any more than I'd say they could specify "when a person with the character class Ranger attacks me" or "when the Dungeon Master rolls initiative".