As I’ve said before, pitching and hitting are very similar. Both rely on building power and speed from the ground and sending it up through the body into the hands.
Check out these hitters lower halves. The hips fire before the top half. The front leg firms up and even contracts as the hips rotate. The front leg’s firmness is what sends all the energy upwards rather than simply rotating round and round. Its the same thing with pitching. That landing leg has to be firm in order to send power upwards and into the arm. Any forward movement after foot strike is direct loss of power.
And to bring it back to pitching. Here’s Halladay. Notice as soon as his foot touches, his knee doesn’t move forward. His leg is a brick wall and as hips come around, it actually contracts and straightens inward.