If one is too inexperienced to do this, then the heel should be used. The pivot point will be the heel, ideally, since this is the point under which the leg connects to the ankle. Some people must rotate using the ball of the foot, however, when after 10 years of training their ankles are still stiff. During the early part of training, the punch should be timed so that it lands when the foot reaches the floor and the muscles of the arm and back contract as the front knee settles into position. Later, the timing of the punch will be varied so that it arrives at the target before and after the step in order to confuse the enemy.
Most people incorrectly time their punch after the stepping action has completed.
This reduces the benefit of stepping forward. The hips are kept square to the front during a stepping punch. Turning the hips during the stepping action slows and weakens the step and does very little to transfer any additional energy into the punch. Wiggling the hips during the stepping action does nothing, and yet it is becoming all the rage among Shotokan practitioners. There is no advantage at all to wiggling the hips and knees in order to set up a sort of vibration based upon crazed jello.