Found this interesting article on biomechanical inefficacies: https://www.thompsonhealth.com/Portals/0/_Health%20Care/Sports%20Medicine/Sports%20Medicine%20Symposium/Handouts%20Biomechanics%20of%20Overarm%20Throwing.pdf
We talk glove arm and arm actions, lower half, head position but what connects the energy one creates from the lower half up through the arm is the positioning and timing of the trunk.
Late trunk rotation is known to be one of the main checkpoints. What I've been looking at is, when foot contact is made and the shoulders are closed, what positions are other parts in? Glove arm is still out front, not tucked yet. The arm is in a semi-cocked position. The hips are open, flexion in the front leg and the back heel is peeled and facing the sky.
Rotation of the trunk follows, with the arm driving into external rotation, the glove arm funneling back into the body (still remaining firm out front, though), followed by the forward movement of the trunk drastically flexing forward, and everything stabilizing into the stride leg.
I understand, rotation is supposed to happen late as I mentioned. I don't know how to teach/cue forward flexion of the trunk. Should it just be a result of complete hip rotation and a "late launch" upper half, or is there dependent qualities that need to be stressed?