I have heard my son’s coaches…on several occasions…tell my son that he should not be leaning back during stride. It is actually shoulder tilt that he has picked up due to leading with the hip. It definitely adds velocity. I did a little research and found an article written by Shingo Mitsumori that stated the following:
I will now explain why one must tilt their shoulder up in order to throw down hill. In a previous article I spoke of torque and equilibrium. Torque is a rotational force and equilibrium is the body’s natural state. Both of these key scientific terms play a huge roll in throwing down hill. Let’s work backwards from release point to the hand break. At release, a pitcher wants their throwing shoulder to be above the glove shoulder, thus creating tilt and a downward throwing plane. As we rewind, the throwing shoulder will gradually flatten and finally the glove shoulder should be above the throwing shoulder. This is what I call the “Seesaw-effect”. The seesaw effect is the result of the body’s natural tendency to return to equilibrium. When the pitcher tilts his/her shoulders at an upward angle in the beginning of their delivery, by the time they get to the release point, the throwing shoulder will end up above the glove shoulder resulting in the seesaw effect thus allowing the pitcher to throw down hill naturally.
Besides creating a downward throwing angle, the seesaw effect, if done correctly, will allow the pitcher to load much more weight on the back side leading to more velocity and less pressure on the throwing arm.
Here is most recent clip of him pitching: