Autotomy is the ability of many lizards (like leopard geckos and crested geckos) to deliberately drop their tail to escape a predator — the tail wriggles as a decoy while the lizard flees.
A dropped tail regrows in most geckos (though it looks different) — but a crested gecko's does NOT grow back. It's stressful and costs the animal stored fat, so avoid grabbing tails and handle gently.