When the handcuffs are fixed, they can be locked and become a double lock state. At this time, the handcuffs cannot be rotated, can't be handcuffed, and can't be opened directly. You need to cancel the locking state first, and then turn into a single lock, and then release the single lock before opening. In the state of double lock, the tenon will not move, which can avoid being easily pried off the handcuffs by tools such as iron wire.