Rule number one of crab handling: make sure it's dead before cutting the string! Ask the crab politely, whilst tapping its legs with a knife or chopstick, "Hello? Hello? Are you dead?"
If the crab nods its head or says, "Yes, I'm dead," beware of the crafty crab! If there's no response and the legs aren't moving, then and only then should the string be cut.
Of course, before you check