[nickrego]

The pitching hand, when the shoulder is turned, can not break from the pitcher's side or back, or it is a feint / Balk.

So basically, the shoulder turn can not change the position of the ball or pitching hand.

If the pitcher SNAPS the shoulder in a way that makes the umpire think the pitcher was trying to deceive the runner, that's a Balk. If the pitcher does any thing that makes the umpire think the pitcher is trying to illegally deceive the runner, that's a balk. Balks are judgment calls.

Personally, I allow quite a bit of leeway on the 'snap' aspect of it, as long as the hand stays still, and the body does not lean toward the base.