When an outfielder is fielding a fly ball and needs to make a throw, to the plate for example, the outfielder will try to get a few feet behind the spot where he will catch the ball and then will move in to it to gain momentum toward the target while catching the ball. Pitchers don't have this luxury on the mound. That doesn't mean there isn't a way to gain some momentum towards the plate.
A common mistake with young pitchers is to simply starting falling forward from the top of their kick toward the plate. Not only is this going to cause a common problem of the arm falling behind in the windup, it will also cause a loss of velocity. The arm will receive little benefit from the body when this happens.
From the top of the kick position the pitcher needs to do a few things that all must come together in order to gain momentum towards home. As the throwing arm drops the pitcher needs to reach down and back towards 2nd base. Again the throwing motion is circular and the pitcher wants the largest circle possible when pitching. Balance is still essential at this point. Reaching down and back with the throwing arm will naturally tilt the front hip up higher than the back hip. The pitcher is now in a position to stride forward toward the plate.
Both legs play an import role at this point in propelling the pitcher toward the plate. The front leg is lowered and driven forward towards the plate in a circular motion that swivels the hips open. At the same time the back leg is pushing off the rubber propelling the pitcher forward to the batter. The combination of these two motions performed correctly will not only add velocity to the fastball but also take pressure off the throwing arm.
It's important to note that most of the power will be generated from the rotation of the hips and a natural stride toward the plate. Many times young pitchers will try to gain velocity by dropping down further on their back leg and driving hard off that leg towards the plate. This will cause fatigue in the pitcher and can also result in the arm not being able to catch up with the body.
The stride should be a comfortable distance, not too short that you're not getting the benefit of your lower body in your delivery and not too long that you land on your heel.
You should land on the ball of your foot with your leg slightly bent. Your foot should land in the same spot on every pitch and be positioned a few inches to the glove hand side of your back foot. This will ensure that you get the full benefit of the hip rotation.
The arm should follow through naturally across your body and the back leg should come forward and land close to parallel to your lead foot. This will put you in a good position to field your position.