Balk called when turning shoulders

[Forest Ump]

Gentlemen:

I need a little help understanding the shoulder turn balk call. I have seen a balk called and I have also called it when a right handed pitcher after coming set, turns his shoulders, rather than his head to look at the runner at first , or for left handed pitcher looking to third. Why would it be OK to turn your head but not your shoulders?

For the last couple of weeks or so I have been brushing up on my rules. I can not find any reference to this shoulder turn balk call in OBR section 8.0 or the J/R. I call Pony and Bronco ( modified OBR).

So what’s the rule on this? OBR and FED. Thanks