Well-intentioned parents and coaches do everything they can to get their young athlete to have success on the mound. The typical course of action is to either get them to mimic their personal pitching delivery because they had success when they pitched the way they pitched, or to mimic a Major League pitcher because they are having elite results the way they throw.
However, the arm belongs to the athlete. Their arm action and arm slot is unique to them and altering their natural arm action can create lack of performance and/or injury.
There are Hall of Fame pitchers that are short (acute angle) on the back side (throwing arm) and Hall of Fame pitchers that are long (straight or obtuse angle) on the back side. There are Hall of Fame pitchers that have an arm slot that is more over the top, and Hall of Fame pitchers that have a very low arm slot, even almost sidearm. Whichever way the athlete throws, let that happen naturally.
What we do want to do is focus on making sure whatever their arm action is, that our front side (glove arm) is mirroring what is happening with the back side to create an equal and opposite affect. We want to look at the relative joint angles, not the overall arm positioning (meaning the arm can be pointing up, down, etc., but the way the elbow is bent is what is important). The joint angle of the glove arm should match the joint angle of the throwing arm. To help equal out the arms, as you are breaking your hands, point your thumb down, and point your palm out with the front hand into foot strike.
The other key part is letting the athlete throw from their natural arm angle. The natural arm angle of the athlete is such that the athlete’s eyes are level at the point of release. If you are unsure of what your athlete’s natural arm angle is, hit them a ground ball to shortstop and have them throw the ball over to first base. Typically, the arm slot they make that throw is their natural arm slot.
Fixing these issues can be tough to unlearn if they athlete has been taught to “throw over the top” and/or "get long or short" with their arm from an early age. Using the National Pitching Association’s Nerve Patterning Drills is a great way to get them back on the right path. It may be uncomfortable at first to unlearn these teaches, but it will be worth the time put in to get this corrected.
Understanding what a critical biomechanical variable is, what a conventional wisdom is, and what is an athlete’s signature is, is some of the many factors that separate Certified National Pitching Association coaches from the pack. The athlete’s arm slot and arm action is an athlete’s signature, we don’t want to mess with it.