The form issue is helped by solid pressure against the rear foot pedal in the blocks.
Many athletes relax that foot and drive solely off of the front leg.
The back leg needs to be firmly pressed against the pedal to allow a quick start. Form also is helped by a low, driving action. Now, onto strength and quickness. Conditioning for the start is helped by Olympic lifts, plyometrics and use of a sprint-harness. Lifting should be done explosively.