A boxer will typically punch the heavy bag while wearing hand wraps and boxing gloves. However, hitting the bag with bare knuckles has certain advantages. Punching the bag without wraps or gloves can toughen the skin while strengthening the bones, muscles and connective tissue of your hands. Following a few simple techniques will allow you to safely hit the heavy bag with no gloves.
Punch the bag with a "flat" fist. Close your hand so that the exposed sections of your fingers are in a flat plane, flush to one another. Both your top row of knuckles and the fingers below should make contact with the heavy bag at the same time. Keep both hands somewhat loose while you are positioned in your defensive stance. Tighten your fist just before each punch strikes the bag.
Maintain a straight wrist as you hit the bag. Ensure that your hand, wrist and forearm are all in a straight line each time you make contact with a punch. Tighten your arm, shoulder, wrist and hand muscles just before impact. Hit the bag as if your entire arm and hand are one solid rod of steel.
Hit the bag with sharp, crisp punches. Pull each punch away from the bag just as quickly as you deliver it, leaving the energy of your strike within its target. Focus on accuracy and technique, as opposed to strength.