Really like the video.
Just some ideas that I have used with my guys. They have thrown hard for their respective ages but the real benefit has been the ability to more precisely locate with near top velocity but seemingly lower effort levels. My experience tells me that young fellows feel strongest across their shoulders and that instructions for max intent will lead to a sort of top down effort. By focusing more on a firm but relaxed (with drive coming from the center) approach we have been able to get a more appropriately sequenced ground up delivery. Less tendency to "fly open".
I feel this kind of rotation is best taught when the posting foot is set up first. The pitcher will have his feet set a comfortable distance apart. Usually close to shoulder width apart. The posting knee is rotated inward, just enough to force the weight to be felt along the insole of the posting foot and centered directly in the inner arch or slightly heelward. The resting internal rotation of the front knee depends on the slope of the mound. A steeply sloped mound may force the front leg to be nearly straight. Once the stride leg is lifted and internally rotated, the pitcher should also feel that the posting hip joint has been internally rotated also, as if there was a stretch along the back side of the pelvis just above the buttocks. The initial drive should feel as if it is coming from the inside of the posting hip socket. It is being initiated by the glute/ham of the upper leg and not the quad. When the drive starts this way, the head will naturally remain in back of the center line of the torso. It should not come from the pitcher rocking his upper body back but from its own inertia. It is not entirely unlike the lay back of the forearm from the inertia of the ball.
One issue with the Hershiser drill is that it can be done by the player just tilting his shoulders back and popping the hips forward. In my opinion, It should be taught with the inward rotation of the hips and drive as described above.
These ideas are not meant to imply that these cues are necessary or even correct. They seem to have worked for some guys.
edited to add: The mounds around here usually get dug out in front of the pitcher's plate. I try to have my guys position their posting foot so that the toe of the foot is level or slightly above the heel. This helps to reduce any tendency to drift towards the pitching arm side and allows the drive to come from the glute and not from the quad.