There are two forces that add velocity to a pitch:

Momentum

Rotational Torque

For momentum to effectively transfer to the ball, the pitcher must use all rotational pivots in order from the bottom up.  The hips must rotate before the shoulders and the shoulders before the arm internally rotates. For this to happen effectively these pivots must be free to rotate completely. Notice the picture of Tim Lincecum at the bottom of the page (Tim Lincecum is a phenom because of his size and ability to reach his top velocity continuously.)  Notice in the picture his weight is slightly leaning to his left. This would be like tilting an open door backwards so the open door slams closed due to gravitational forces. This gravitational pull is helping to create full range of motion in Tim Lincecum’s hips and shoulders at front foot strike. If he or the door was tilted the opposite way then these gravitational forces would work against his momentum by decreasing full range of motion in his rotational pivots. Using the force of gravity to increase the range of motion in your hips and shoulders will have a significant effect on your velocity. This is a big reason why Tim Lincecum can throw so hard for his size. He is working with the forces of nature to generate his power.

If you study the animated image here of Tim pitching you can see clearly the effective transfer of momentum through his rotational pivots. Watch his front leg land and his back hip rotate all the way around as his back leg triple extends. From here the momentum moves into the core because his front leg has stabilized  and his weight is being held back because his back shoulder is waiting for his hips to open to the target. This forces the core to tighten because the hips are rotating before the shoulders. His core looks like a rag being rung out or a rubber band being twisted at this point in the delivery. After this tightening of the core the momentum travels up into the shoulders. This torque pulls the back shoulder around and he sets the fulcrum, for the rotating shoulders, with his glove hand over his front leg.  The front leg continues to stabilize as his weight begins to shift over his front knee allowing the momentum to transfer into the final pivot. This is the shoulder pivot or the rotator cuff. Notice that when his trunk is fully forward, his arm is completely externally rotated. Now the arm fires like a rubber band and begins to rotate forward as also all the momentum from the body jumps into the ball like a passenger riding in a car and hitting a brick wall at 100 mph.

What Tim Lincecum continues to teach us is how to pitch with the entire body and that the arm is only along for the ride. This is exactly why little guys can throw so hard and old pitchers can still compete. Tim Lincecum uses gravity to aid momentum and his momentum to build torque in all of his rotational pivots. He also fires those pivots in the perfect order at the perfect time for effective momentum transfer. Everytime Tim Lincecum pitches, you should be watching because it is a lesson in Top Velocity.