Carl Lewis is arguably one of the great runners of the 20th century, especially in form. Even he however, does not have all the components of great technique. Carl Lewis has good form with a few simple flaws. If you notice on the starting blocks, Carl Lewis pushes-off, completely extending his leg to no avail. When on support at the starting blocks, the majority of your body-weight rests on your hands. When you release your hands, your support shifts to your feet. This steep angle of incline cannot be maintained while running. It can however be used in order to gain momentum and speed very quickly. It is an advantageous position of acceleration because you do not have to lean into your run on the contrary, you start your run with a tremendous lean. Once you are airborne, how much can you push-off off the ground? Not very far. Try to imagine pushing out of the starting blocks similarly. What good does it do to extend the support foot if there is no body-weight to back it? All of your body-weight is now airborne, the only reason you still have a foot on support is because you arent moving fast enough to pull it off of the ground. Knowing that, there is no reason to exert any special efforts to push-off after the starting blocks. The focus needs to be on shifting support as quickly as possible: pull pull pull.