One of the “That makes sense” moments of the Rotary Swing is their concept of centripetal force - pulling versus pushing. Learning to use the back muscles really helps to feel this pulling notion.
You can utilize both arms being straight initially in the back swing to assist in moving the right shoulder back, in a piston like fashion, and helping to complete a full 90 degree shoulder turn.
Again, we need to be proactive. The fewer variances we add early on in the swing, the more consistent and predictable we will be. We want to be as wide as possible throughout the backswing. The hips only turn about 30 degrees.