This is a punch I use often, and especially against a slightly shorter fighter. And exactly as Salty describes.

I basically try to pin a guy in the corner applying pressure, then wait and watch his feet or legs for a balance shift, signally he`s making a move out punch...then just release. He is usually throwing a left...but sometimes a right. Either way..I basically just unload an overhand right, or straight right as soon as I notice the shift in weight beginning.