Double locks
Handcuffs with double locks have a lock-spring which when engaged stops the cuff from ratcheting tighter to prevent the wearer from tightening them. Tightening could be intentional or by struggling, when tightened the handcuffs may cause nerve damage or loss of circulation. Also some wearers could tighten the cuffs to attempt an escape by having the officer loosen the cuffs and while the cuffs are loose attempt the escape. Double locks also make picking the locks more difficult.
There exist three kinds of double locks as described in a Smith & Wesson brochure:
Lever lock
These are double locked by fully lifting the lever with a fingertip and then allowing it to return. This causes the lock spring to move into a position that locks the bolt thus preventing the cuff from being further tightened. Thus no tool is required to double lock this type of cuff.
Push pin lock
These are double-locked by fully depressing the push pin using the small peg on the top of the key. This causes the lock spring to move into a position that locks the bolt thus preventing the cuff from being further tightened.
Slot lock
These are double locked by inserting the small peg on the top of the key into the double lock slot. In this position, the small peg can contact the end of the lock spring. The key is then slid towards the key hole. This causes the lock spring to move into a position that locks the bolt, thus preventing the cuff from being further tightened.