Just about everyone knows how a pair of handcuffs work: You can push the locking arm through the mechanism until it passes right through and the cuffs are open. By default, handcuffs are locked and the locking arm can only move through the cuffs into the closed position. The locking mechanism is free to move, but only into the closed position.

Double locking a pair of handcuffs prevents the locking mechanism from moving in either direction, therefore stopping the locking arm from moving and making the closed cuff tighter.

A standard handcuff key has a short barrel with a single tooth at the end for unlocking the cuffs. However, at the opposite end of the key you'll find a short, blunt spike. This spike is used to double lock the cuffs.

On each cuff is a small hole in the locking mechanism housing. The hole is usually on the side of the cuff, perpendicular to the side where the keyhole is. To double lock the cuffs, simply lock the cuff on the wrist to the desired tightness and then push the short spike on the key into this hole. The spike pushes a catch into the locking mechanism. The cuffs are now double locked.

If you've ever watched "Cops", you've probably noticed that when they slap the cuffs on a suspect, they get out their key and seem to mess with each cuff. They're double locking the cuffs so they don't get any tighter when the suspect sits in the car, and leans into the cuffs behind his back.

To unlock a set of double locked cuffs, you insert the key into the keyhole and turn it in the opposite direction that opens the cuffs. This releases the double lock. Turning the key back in the other direction will open the locking mechanism.

Double locking handcuffs is important when securing a prisoner as it makes it more difficult to pick the relatively simple locking mechanism. It also prevents the cuffs from getting tighter and thereby reduces the risk of causing nerve damage in the wrist.

My Dad's a cop
My brother's a cop

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