[Par71]

Pectorals - they pull the shoulders and arms forward (2-M-3)

The problem with the Shoulders is that they can move independently from each other. For example, you can turn the Right Shoulder back without moving the Left Shoulder (a false feel Shoulder Turn). I think that's what Homer is referring to when he says "moving the Right Shoulder, especially, is a non-essential variable". When you keep both Shoulders forward you restrict that independent Shoulder motion, and both Shoulders should turn in sync.

In addition, keeping the Shoulders forward gives more room to the Arms.

And I find that it helps to stay on plane.

I use it a great deal.