At first glance, the overhand looks like a wild and uncontrolled punch. While it is true that boxers do throw haymakers from time to time, the overhand, usually referred to as the overhand right (orthodox stance), is a power punch that is very controlled and usually timed. It is most effective when a shorter fighter is throwing it against a taller opponent.
The overhand is thrown with your rear arm. From your neutral stance, lift your rear arm out a few inches then up and over towards your opponent's head. The motion should follow the path of an arc. Be sure to get your shoulder into the punch for added power. Do not over extend the arm, do not drop it either. And remember to keep your other hand on your face for protection. And once again, return the arm back to it's original position.