Round kicks are the best way to engage your core and to help build combinations to keep your rhythm going during a good heavy bag workout. This is going to be the exact opposite of the punch. The energy is going to flow from your shoulder to your hip to your leg. Concentrating on twisting your shoulders will really help the recoil of the kick setting you up for the follow up. Make sure that you are hitting with the instep of your foot and not your toes. I try to let the curve of the bag connect with the curve of my ankle and foot. Remember, your not trying to kill the bag, just work with it.