A pitcher that goes forward with his front leg bent carries more potential energy (forces of gravity acting on a body) into front foot plant and consequently more momentum into hip and shoulder rotation. The pitcher that goes up and back down with his leg lift before going forward is essentially giving up that potential energy he’s created and might as well not lift at all.
That said, this is a significantly smaller portion of the generation of velocity than the rotational energy of hip/shoudler separation and should only be maximized to the point of not losing balance or posture.