Since handcuffs are only intended as temporary restraints, they are not the most complicated of locks.This is why escaping from handcuffs is a common stunt performed by magicians or skilled criminals, perhaps most famously Harry Houdini.
There are ways of escaping from handcuffs:
1. slipping hands out
2. lock picking
3. releasing the pawl with a shim
4. or simply opening the handcuffs with a duplicate key, often hidden on the body of the performer before the performance.
The above methods are often used in escapology. As most people's hands are larger than their wrists, the first method was much easier before the invention of modern ratchet cuffs, which can be adjusted to a variety of sizes. Modern handcuffs are generally ratcheted until they are too tight to be slipped off the hands. However, slipping out of ratchet cuffs is still possible. During his shows, Harry Houdini was frequently secured with multiple pairs of handcuffs. Any pair that was too difficult to be picked was placed on his upper arms. Being very muscular, his upper arms were far larger than his hands. Once he had picked the locks on the lower pairs of handcuffs, the upper pair could simply be slipped off.
It is also technically possible to break free from handcuffs by applying massive amounts of force from one's arms to cause the device to split open or loosen enough to squeeze one's hands through; however this takes exceptional strength (especially with handcuffs made of steel). This also puts an immense amount of pressure on the biceps and triceps muscles, and when tried by suspects (even unsuccessfully) can lead to injury, including bruising around the wrists, or tearing the muscles used (including pulling them off their attachments to the bones).
Another common method of escaping (or attempting to escape) from being handcuffed behind the back, is that one would, from a sitting or lying position, bring one's legs up as high upon one's torso as possible, then push one's arms down to bring the handcuffs below one's feet, finally pulling the handcuffs up using one's arms to the front of one's body. This can lead to awkward or painful positions depending on how the handcuffs were applied, and typically requires a good amount of flexibility. It can also be done from a standing position, where, with some degree of effort, the handcuffed hands are slid around the hips and down the buttocks to the feet; then sliding each foot up and over the cuffs. These maneuvers, and the reverse (otherwise impossible) maneuver of bringing the handcuffed hands up behind the back and forwards over the head and then down in front, can be done fairly easily by some people who were born without collarbones because of the inherited deformity called cleidocranial dysostosis.
From this position, one has a better chance of attempting to use a tool (such as a shim or lockpick) to work one's way out of the handcuffs.