Hi,
shimming is a technique mainly used for bad padlocks, it works on handcuffs but only in cases where no double lock is applied, which never should be the case in real-life scenarios
For handcuffs it means to put a thin piece of metal between the bow and the ratchet of the cuff, so the bow can slide out without being grabbed by the tooth of the locking mechanism. This of course only works when there's enough space, on very tight handcuffs one would need extremely thin metal.
There might be cases in which it's easier to remove the double lock than opening the cuff, in those shimming really could help escaping a correctly applied cuff...
Unimuc