When most people think of someone getting out of handcuffs, they think the prisoner either had a key or got out of the cuffs by using something in the key hole.
A majority of the time escapees use a thin piece of metal or a paper clip to get between the teeth and the locking mechanism. I have also seen it happen where the end of a comb was used to push down on the locking mechanism (between their wrist and hand) and rolling the wrist out of the cuff.
If the handcuffs are double locked it makes it more difficult, but double locking can be overcome by slamming the handcuffs down (or up) on something solid to pop the double lock off.