Throughout our years of coaching and instruction, we have heard from parents that they are sending their son or daughter to a particular instructor who has quite a resume; some pitching in college and some at the professional level. Personally, I believe that practical experience does make a difference. However, we always ask if the instructor is videotaping their child. If the answer is "no" then it really doesn't matter how high up the ladder the instructor climbed in their career. You are not getting your monies worth. 

The human eye simply cannot isolate all the nuances of the complex pitching delivery, arguably the most difficult motor skill in all of sports. Consequently, videotaping is necessary for an instructor to properly diagnose mechanical flaws. Furthermore, visual feedback is necessary to effectively convey the diagnosis and prescribed remedy to the athlete. 

Below, I've excerpted a segment from (November 24, 2014) that supports our claim. 

Coull, J., Tremblay, L., & Elliott, D. (2001). Examining the specificity of practice hypothesis: Is learning modality specific? Research Quarterly for Exercise and Sport, 72, 345-354.

The specific of practice hypothesis was examined using a tracking task. College students (M = 12; F = 28) were Ss and presumed to be novices at the specific task. Two experiments were conducted.

First, visual and auditory feedback about performance was provided. Vision was deemed more useful than hearing in the early stage of acquisition. Performance gains were retained when no feedback was provided. Learning was specific only in the visual learning condition.

Second, visual feedback and auditory feedback were combined. Similar results to the first experiment were revealed. Vision appears to dominate audition in the learning of motor tasks.

Implication. When learning a task, visual cues should be emphasized to produce better instruction.