You know the best answer to this question is "that depends on the boxer". I have seen some fighters that look like they were doing a hula dance when they pivot on the back foot to add weight to the punch, and I have seen some boxers who just ever so slightly keep the heal off the floor to assist in the pivot, and some fighters keep the back foot flat and still manage to rotate their hips and body into the punch. Most of the time you are moving and when that split second opportunity opens up for a good power punch, you are usually not in the exact position to do the classic heal off the floor and pivot move. Some good fighters usually develop quirky and goofy techniques that work for them and them alone..and I finally got smart enough to let them go with what works best.