For a punch, what i usually do is have my elbow in a contracted position, and the punching arm's pec is relaxing, then when i twist my body, the arm will swing faster. I only have my arm contracted a couple of frames before i hold the elbow and contract the pec, this way the arm will extend using it's force that was created with the initial swing, then it's just where on the body i'm aiming at, and whether my arm is out enough for a solid punch, if it's still contracted, i'll extend/relax it. I'd have to go in game to show how I punch, i guess everyone has thier own way of doing those kinds of things.

Hope this helped!