what are some specific flaws that cause injury to the shoulder?
Things that can lead to injury are timing problms (e.g. opening up early), postural problems, incorrect throwing technique, and overuse.
How can you work on NOT opening up early , and fixing those timing problems?
I’m sure you’re going to get lots of advice on the actual physical effects of now doing-this … not doing-that, but one of the most common contributors to injuries during amateur play is the surface that a youngsters pitches off of.
Flat ground, domes, and mounds that are poorly maintained, not surface integrity - usually composed of sand, lose top soil and dirt, large holes in front of the rubber and large holes right where the stride foot lands, wash-out slopes that pitch off to the left or right, and other conditions will do nothing for any style or movement that you’re trying to perfect.
A simple practice session on any and all pitching surfaces that your going to encounter during amateur play is one of the techniques of training yourself to adapt and compensate.
By the way, if you really want to test the metal of a person that calls himself/herself a pitching coach, ask this person to coach you on adapting to several of the conditions that I just stated above. If he/she doesn’t have a reasonable answer for you, perhaps you should re-think the benefits are of listing to this coach.
Throwing with your hand outside the ball also causes strain on the shoulder…I call it “leading with your pinky”. It can cause a cut movement to the ball but it really does play heck on the posterior side.
Throwing cold can cause injuries, also.
Well, things that can lead to timing problems are posture problems, lack of glove-side control, and lack of tempo/momentum. For posture issues, work on keeping head and spine upright. For glove-side control, get the arms into an equal and opposite position and keep the glove out in front of the chest until ball release. For tempo/momentum issues, work on getting the hips moving forward sooner/faster.