[Yoda]

I associate tracing with actually "covering" (drawing over tracing paper the original marks to reproduce exactly the lines) which I know is to be distinguished in TGM...(?)

Your concept is correct, woodandcoal.

If the Clubshaft was a laser beam, then the orbiting (and tracing) Sweetspot would, in fact, simultaneously 'point at' and cover the Plane Line. Consequently, there would be no Arc of Approach (to the Plane Line) and hence no curved 'Visual Equivalent'. Then, the only available reference line for 'On Plane' Clubhead Delivery would be the Straight Plane Line itself.

But...

The Clubshaft is not a laser beam. Its length is finite, and it cannot grow longer during the Backstroke or shorter during the Downstroke. Therefore, the orbiting Sweetspot can only 'point at' the Straight Plane Line as it 'covers' its own true curved Path.

Now the golfer has available two Delivery Lines: the true geometric Plane Line (to 'point at') and its Visual Equivalent, the curved Arc of Approach (to 'cover').