the balk...

[Sal Giaco]

It is my understanding that in college and OBR, the front shoulder must remain perfectly still. Generally speaking, I have rarely called a balk for turning the shoulder (I just don't see pitchers taking advantage of that as much). The move they seem to be teaching at the college level is "buckling the knee". I believe the baserunner locks in on the pitcher's front knee to see the first movement towards home. Pitcher's are taught to slightly flex their knee followed by a quick throw to first base.

I believe your question pertained to higher levels of amateur baseball. Gee brought up a good point that there really is no need to turn the shoulder to see first base because if that was a concern, the pitcher could just "open" up his set position. Therefore, don't give the pitcher "an inch" because he will inevitably, look to exploit that advantage. Moreover, if you allow some movement, then you put the pressure on yourself by having to decide how much of a turn and how fast is acceptable. Again, the higher the level of ball, the less this should become an issue.