They say to relax your shoulders, but how do you keep from flapping your arms when you punch?

I have always heard to keep your whole body "loose and springy". I just took it as relaxed. When you throw your punches swing like your slaming a door or something, turning your body into it. Right before you make contact, tighten up. clench your fist and arm muscles just before contact. try it on the heavy bag. You will be able to hit harder with less effort.

thanks,
When a guy is told he is pushing when he punches, but the punches look hard,what is happening? Is he muscling it too much?

I dont want to lead you astray here, but I would take that as not being able to turn on your punches.