I provided a private lesson to an American middle school baseball player yesterday.
It was hitting session, and I usually give him some of the exercises for increasing upper body rotation.
I think most of younger baseball players have enough mobility for upper body rotation, but they are not sure how to move properly.
That’s why we (coaches) need to learn proper mechanics and constantly explain to younger players.
And one of key things that I think is “Rotate your shoulder to shoulder under your chin”.
For that purpose, sufficient thoracic spine rotation is necessary.