In addition to what I just posted - video, there are things that are overlooked with coaching and managing pitchers. Those things are:
Regardless of age and maturity, these topics all play important roles with evaluating the physical stamina and healthy state of the human body while performing. Unfortunately, youth athletics uses a benchmark of - do this, do that, compared to mature professionals. Form and style is a mixture of fully developed men who somehow are an overlay, or template if you will, on a youngster who has none of the benefits of growth and experience of the professional.
Capability is so dependent on what there is to work with that ... somehow coaching gets dumped in the blender.. hit puree boys, and walla.. a perfect smoothie, one pitcher deluxe. A human being under the age of 18, just doesn't come around to holding his/her own like others his/her age on the field of athletics. Youth has it's limitations, despite all the wants and desires on all sides wanting better.
Tolerances impact the mental and physical stress is unique to all, and that varies from day to day. Unfortunately, the amateur ranks rarely has the professional coaching that can monitor tolerance. Tolerance - in my opinion, can indicate just how well, or not, coaching and performance come together.
Longevity is the litmus test of how everything comes together. Now this doesn't mean strike out, after strike out. Longevity is witnessed by the smooth fluidness of delivery after delivery during an appearance.