[tongzilla]

But if the left palm is on the plane, and the left shoulder is not, then neither is the left arm, hence there must be an angle between the palm and the arm - how can the left wrist be flat?

Thanks,
Chris

Oh, I get it. Your problem lies in not understanding the Plane!

Please go to 10-6-B #2 now. You can see the Left Palm is Turned facing toward the Plane as per Standard Wrist Action. Is the Left Shoulder or Left Arm On Plane? Clearly not! However, the Left Arm Flying Wedge Assembly is still maintained -- the Left Wrist is Flat.

(Now, you should not use a photo for a purpose beyond its captioned intent. And I rarely do that. But this is the best photo I could find to illustrate my point.)