[6bmike]

10-13-C says that this Shoulder Turn moves the shoulder "at right angles to the spine." At what ANGLE do you think a FLAT 10-13-B Shoulder Turn moves in relation to the SPINE?

IMBW but:

Regardless of spine angle, all shoulder turns will rotate at a ninety degree angle to the spine. It may look like the left shoulder dips with a greater spine angle at address but it is still perpendicular to the backbone. Likewise a flat turn will look like it is the only 90* rotation but it isn't.