If you are polymorphed into a troll and then killed, the game immediately checks the random chance of whether you will automatically revive. If you would not, you simply return to your normal form (or die if you’re wearing unchanging). If you would, you lie dead for a little while until you do revive. This could work via the mechanism that lets you mimic a pile of gold, except you are mimicking a troll corpse instead. If a monster disturbs your corpse prior to your revival, you “die” - for real if you had unchanging, or returning you to your normal form if not.