The public's fascination with one's ability to break free of handcuffs likely stems from the assumption that, since handcuffs are used to restrain potentially dangerous criminals, they must be inescapable. The fact of the matter is handcuffs contain a very simple locking mechanism that's basically gone unchanged since their invention in 1862 --- so the restraints conquered by today's escape artists aren't a whole lot different than the cuffs and shackles Houdini mastered before the turn of the 20th century. The reason so many magicians have been able to accomplish what countless lawbreakers have not is a simple matter of positioning: people who are arrested are cuffed with their hands behind their backs, while magicians are almost always handcuffed in front.
While picking a lock might seem like a daunting task, all one needs to unfasten a pair of handcuffs is a common bobby pin --- and a little patience. By inserting a straightened bobby pin into the keyhole and bending its tip left and then right to form an "S" shape, you'll convert it to a universal lock pick that could help you free yourself of most types of cuffs. Simply insert the bobby pin/lock pick in the keyhole and turn it clockwise. If the handcuffs don't open on your first attempt, continue turning the makeshift key at different depths in the lock until it "catches" and releases the cuff.
Designed to make restraining someone a quick and uncomplicated process, a pair of handcuffs features two rotating arms with serrated edges that easily pass through the locking mechanism when turned in one direction, but catch on an inner ratchet when turned in the other. When handcuffs are in use, the wearer's wrists block the rotating arms from being turned in the non-ratchet direction; so the only way to remove the cuffs is to disengage the ratchet. While this is supposed to be done with a key, enterprising escape artists discovered early on that it's often easier to bypass the locking mechanism. By inserting a flat metal shim --- or even a clip broken off of a ballpoint pen --- under the rotating arm's teeth and applying pressure to the ratchet, one can disengage the locking device and open the cuff. The real talent in utilizing this method is using your mouth to hold the shim while disengaging the ratchet.
Because the public's confidence in the restraining power of handcuffs is so high, it's not uncommon for magicians to use trick handcuffs that are no more sturdy than the plastic cuffs you might have played with as a child. Among those used by professional entertainers are specialized handcuffs that open with the touch of a button, rubber handcuffs that can literally be stretched around the hands and cuffs in which the serrated edges have been filed down.
For all his skills, even Houdini occasionally came across a pair of handcuffs that presented too much of a challenge. Whenever this happened, he'd claim the cuffs were too easy to escape from and would insist that more pairs be added to make the feat more interesting. The added restraints, which were trick handcuffs from his own collection, would then be secured first to ensure the difficult handcuffs would be placed higher up on his arms. Then, after easily freeing himself from the trick cuffs, he'd slide the difficult handcuffs down from his much thicker forearms and over his smaller wrists and hands.