the heavy bag is the one punching bag you cannot do without; for many years it was just about the only bag. This is where you practice each punch until you perfect it, develope the "punching muscles" in your back and shoulders, condition and strengthen your hands, wrists and forearms, learn to punch in combination, judge distance and learn how to move with/around a body. Your bag should always swing and you should always play it like its real. By that I mean moving in, out and about to set up particular sequences of punches, and above all do not get in the habit of stepping in throwing a combination then stepping straight back to get out, or dropping your hands and standing there. The bag and its movement offer a perfect chance to learn how to step in , punch , then turn an angle to where you can't be hit but are still in range to prolong the exchange (robinon v Gatti, 1 & 2)