[Bigwill]

Calling Annikan Skywalker,

I believe the left arm connects to the shoulder by a "ball in socket" joint because of this any motion of the left arm back and through will have external and internal rotation (not always in that order) whether you are tracing a straight line or not. The amount of rotation may be very small but there will be some.

I will wait for someone with a much better understanding of biomechanics than I to post and confirm or deny my conclusion above.

Sorting Through the Golf Nut's Catalog.

B-Ray

The shoulder is a ball and socket joint, but it's held together and supported by soft tissue, not the ball and socket structure itself. Because of this, it is the most freely movable joint in the body. The movement of the left arm back and through, and the rotation of the arm, are independent of each other. So the arm dosen't necessarily have to rotate as the arm moves back and through; those two movements need to be coordinated.

That said, I think that it is probably unlikely that the arm wouldn't rotate to some degree, due to the movement of the club during the swing, unless the player does something (either intentionally or through poor mechanics) to prevent it.