There is a lot of controversy around the glove side to pitching mechanics. Conventional Wisdom would coach the pitcher to pull down or pull around the glove side to launch the throwing arm into action. The problem is this would go against pure speed and classical physics.
The reality is that the glove arm to shoulder must act as a fulcrum for the shoulders during the throw to allow for efficient speed mechanics. To understand this we must first define the fulcrum. A fulcrum is a pivot about which a lever turns. The lever in pitching mechanics is the shoulders and also the hips but in this article, we are only talking about the shoulders. The shoulders must swing like a door towards the target. Once they open then the arm must launch over the top of the door. If the pivot or fulcrum of the door is moving when the door is slamming closed then the door will not reach its top velocity. The same results would occur with other tools that use the fulcrum or pivot to swing a lever. Good examples similar to pitching, which I have used on this site, would be the catapult or mousetrap.
What is the best way to use the glove side to increase shoulder and pitch speed?
Before I go into answering this question please understand that there is several critical mechanical components that occur before the glove side even comes into play during the pitching delivery. It is essential when learning velocity focused pitching mechanics that you master all of these critical components before front foot strike which is when the glove side takes action. You can learn all of these mechanical components in the 3X Pitching Velocity Program.
When the glove side is ready to serve its purpose, it must immediately spring into action and become an effective fulcrum for the shoulders to launch. This means it must tuck tightly under the glove arm and shoulder. The tuck should be enough for the chest to push forward while the glove arm bicep is fully contracted like when curling weight. Do not let the glove drop or the elbow to swing out towards the dugout. The elbow must stick into the obliques of the core. By tucking the glove side up and under the arm and keeping it tight with the chest pushing forward this will set a strong fulcrum for the shoulders to efficiently accelerate around. Remember the purpose of the tuck is to create a stable fulcrum or pivot, NOT to assist the throwing arm during the launch. Using the glove side to assist the throwing arm during the throw will cause instability in the pivot point, slowing down the speed of the shoulders and could also cause arm drag which can lead to elbow and shoulder injury. It can also cause the shoulders to throw the ball early reducing hip to shoulder separation at front foot strike.
The best way to learn the glove side is to watch the glove side of hard throwers in slow motion.