One of the crabs natural escape mechanisms allows it to drop a limb in order to avoid capture. This is known as autotomy. Should a predator clamp down on a crab's claw or any other limb preventing its escape, the crab may self-amputate or sacrifice the limb so that it can free itself and swim to safety. When you pile a bunch of crabs into a pot and turn up the heat, the crab instinctively tries to flee. Dropping its claws is simply the crab's last futile attempt at escape.

To avoid this, you can stun each crab by immersing it in ice-water for several minutes prior to cooking (this causes the crab to enter a dormant state, or sleep.) Or, you can euthanize (kill) the crab immediately prior to cooking. This is done by poking a sharp, pointed object (e.g., an ice pick) through the crab's head which kills it instantly. Hold the crab on its back and insert the pick through the shell just below the crab's mouth parts.