[Mike O]

Other questions you may want to consider...
Why are some center lines drawn as curves?
What is Homer trying to show by drawing these curves?

The curved center line solves the basic problem of trying to draw 3-D figures on paper.

Take a golf ball. Draw a circle around the equater of the ball. Now look straight at the ball with the line facing you. You should see a straight vertical center line.

Now rotate the ball clockwise a little (45 degrees or so) around the horizontal axis. This is represented by the curved vertical center line at impact in 2-C-1 #1 and #2A and #2B.

Similarly, if you look at a Cut Shot (which is just a shot using Vertical Hinging) in 2-C-1, you can see the ball never rotates around the horizontal axis since the clubface remains square through the shot. However, because of the laying back motion of the clubface, this causes the ball the spin back a little between impact and separation, as you can see with the curved center line in 2-C-2 #3.

Probably- rotate back versus spin back would be a better choice - as that term of spin might mislead some but that's just my two cents- good post leo.