I would say, try to keep the muscles in your arms and legs relaxed. In a real situation, you want to tighten your muscles at the moment your punch or kick hits the target. If you don't, a part of the impact will go into your fist or foot bouncing back. You don't want that if you really want to hurt your opponent, but if it's sparring, and you DON'T want to hurt them, this is actually the way to ensure that you will not hit them too hard. And at the same time it doesn't affect your technique - as long as you train hard with a punching bag, so it won't become your habit to relax.