[Yoda]

Good place to for someone to explain why tracing a straight plane line in the backswing is Imperative?

Interesting point, nevermind, and one that deserves an important differentiation.

Tracing a Straight Plane Line on the Backstroke simplifies the mission of getting to the Top correctly. However, because the Top itself can be (and is) a re-alignment place, the route chosen from Address can and does vary widely.

On the other hand, Impact is not a re-alignment place. You are either on the money there or you are not. And Tracing a Straight Plane Line on the Downstroke is mandatory to correctly Deliver the Clubhead into the Ball.

Therefore, it is this last Tracing -- not the first -- that is The Third Imperative.