"...except that you have to push off the rubber".
It was a little ambiguous to me which one of you thinks that pitchers must push off of the rubber. Whatever, the one of you who apparently believes that pitchers push off the rubber may wish to re-think his understanding of mechanics and physical reality.
This topic has been debated almost endlessly, but there is really no debate at all--unless you want to spend lots of time redefining the meanings of perfectly useful words.
"Push" is a useful word in the context of what happens mechanically during a "push-up", correct? At the beginning of a push-up, the athlete must expend just exactly enough force to counterbalance the gravitational force--i.e., there is zero net force, no movement, until the athlete exerts a greater-than-gravitational force to accelerate his body upward. When the isometric "push" upward exactly counterbalances the gravitational "push" downward there is no net force on the body, no acceleration. When most people use the word "push" they are really talking about application of a force needed to overcome our equilibrium with gravity, leading to acceleration. If you buy that definition for the moment, then think about the following:
When an athlete does a push-up, what happens to his arms? The forearm-elbow-upper arm angle of his arms change. At any time this angle is not changing, there is no "push" other than that required to exactly counterbalance the gravitational force.
Thus, no net force, no net "push", is being applied during a push-up unless the body is accelerating. The way we are built, our elbow angles must change to accomodate the "push" during a push-up.
Same is true for a pitcher's post leg and its shin-knee-femur angle. If there is an accelerating force applied accross this biological hinge then its angle will have to change to accomodate the acceleration. If the hinge angle doesn't change--there was no net force, no acceleration, no "push" in the sense that people normally use the word.
Study good video of pitchers' motions. You may need to study video from several perspectives and you may need to look at it in slo-mo to assure yourself of this: Pitchers do not generally "push" off of the rubber. I suppose they could do that if they wanted to, but it would necessarily lead a straightening of the post leg and that doesn't happen. When a pitcher starts very tall, the angle in his post leg will generally decrease during the stride forward--the exact opposite of "push"--and his post-leg will pivot underneath his pelvis as gravity pushes him forward with his stride leg off of the ground.
Pitchers who start lower, with a smaller angle in their post leg, maintain this angle through footstrike and follow-through. If they "pushed" off of the post leg--that is, if they actually applied a net positive force with their post leg--then that leg would have to straighten and they would be seen to pop-up or jump. That is not what pitchers do, at least not the good ones.
Nevertheless, some otherwise outstanding pitchers and coaches have misconstrued the concept of "push" and thereby given it a wrongful place in the coaching lexicon in their attempts to describe what they thought was important about good pitching mechanics.
It is okay to respect and even revere the accomplishments of outstanding pitchers and coaches without passing along the flawed parts of their wisdom.