Hoysauce got it right, he doesn't push off the rubber and neither does anyone else (unless their post leg straightens out and they exhibit some type of a 'jump' in their motion).
Try to get this right, pitchers, it is shocking that so few of you understand the factual details of your own mechanics.
If there is a slight bend at your knees in your starting position, and if your post leg never straightens during the stride forward and release of the baseball, then you are not pushing off. Nor should you push off. If you did, the pushing force would cause you to jump.
Undoubtedly, some coach back in the day decided that "push off the rubber" sounded like good advice--if you're pushing, that means you are working harder and applying more force and that's good for pitchers, right?
Sorry, no amount of repeating nonsense is going to make the nonsense turn into fact. Pithers do of course need to generate lots of force, but it doesn't come from "pushing off" of the rubber.
When a pitcher shifts weight toward his front hip and simultaneously lifts his stride leg, the gravitational force acts on his body like a lever (with the fulcrum being his post foot). As the pitcher begins levering forward his post leg swivels underneath as his hips rotate open.
Look carefully at lots of video of pitchers--the post leg does not straighten out at any time between start of motion and landing of the stride foot. With only a single camera angle, there can be the illusion of "straightening" as the bent post leg swivels through your line of vision--nevertheless, there is no actual straightening of the post leg and therefore no "push off the rubber".
The sensation that you may mistakenly call "pushing off" is really just isometric tension against the gravitational force--that is, you must keep just enough tension in your post leg to keep from sinking to the ground in normal gravity. Since you lift one leg off the ground during your stride, your post leg must then maintain all of the isometric tension necessary to keep you from falling. No doubt, that can feel like a "push off"...but it isn't.