I think there was a real thread awhile back.
In any case here is a good video
https://www.youtube.com/watch?v=7CTUx4Z ... ata_playerThey are basically just a ratchet with a spring loaded pawl. If you push something between the teeth and the pawl you can shim them. The video kind of skips over the fact that real handcuffs will have a double lock feature to prevent them from being closed further and injuring the person being cuffed. If the cuffs are used properly the double lock will stop you from shimming the cuffs. The double lock is basically just a piece of metal that blocks the pawl from being retracted.
As a fun fact the London, England, emergency services reported a 50% increase in handcuff related calls a couple of months after 50 shades of grey came out.