In the heavy bag training in career mode, start by a throwing a right hook to get the bag going, then throw a left hook while the bag is moving to the right. When the bag moves left, throw a right hook, etc. The bag will swing more and more, and you will receive the swing bonuses, increasing your score. Use good timing, and hit the bag when it is just entering the upswing. You will get more attribute points to increase your fighter's punch power and speed.