I'm not anywhere near as intelligent or scientific as most people who make a living in pitching instruction. However, I have worked with several kids who ended up pitching in the big leagues.
Because of not being very smart, it made me instruct in "hick" mode. There are many who are much more knowledgable and I don't know if they would agree to what I say here.
We often talk about mechanics and arm action. We talk about the legs, the arm, the shoulders, the hips, the head, the stride, the landing, and everything else.
Just in my own personal experience I've had the most success (by far) using the "belly button" as the single most important key. (both for control and velocity improvement) While there are lots of important things involved in throwing a baseball IMO the two most important are natural ability and the "belly button".
Not trying to be an expert here... just saying I've had significant success getting kids to throw with their stomach being the main focus. Remember that the hips and stomach work together in many ways. The belly button is just an easy point of reference.
Good coaches will understand how this might work, without getting into details. And of course, I expect some debate. I don't do much instruction anymore, but if I did kids would hear "throw with your belly" a lot.
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