The more relaxed the muscles are, the more energy can flow through the body. Using muscular tensions to try to 'do' the punch or attempting to use brute force to knock someone over will only work to opposite effect.
Bruce LeeIf your opponent is at a distance, kick him in the groin. If he gets close, poke him in the eyes, bring up your knee, pop him with an elbow, dig a corkscrew punch to his stomach.
Bruce Lee