ATTENTION! This method is impossible if your hands are at your back with the keyhole facing away, as is the case when a law enforcement officer makes an arrest.
Watch the video to see it in action.
Step 1: What You Need...
2. One ordinary bobby-pin. Remove the plastic end.
Step 2: How to Do It
Take it out, and bend it the other way. and you should end up with an angle shape shown in the picture.