According to Jens Pulver, if you want to improve your punching power, it can help to strengthen your hands. The more muscular your hands are, the heavier they are and they harder they hit when they land. Also, strong hands are a tremendous asset for ground combat. The stronger your hands the harder it is for your opponent to break your grip. In this video, Jens Pulver shows an unorthodox way to strengthen your hands. He does pull ups with tennis balls in his hands. Check it out.