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.

In view of this, gradually floor handcuffs began to be used. Because the connection of the handcuffs is connected by two or three iron rings in parallel, the distance between the two handcuffs becomes quite short, and the wrist can not be freely rotated back and forth, so the restrictions on movement become very large.