In regard to the above illustration of 1-L-18. Assuming plane shifts and looking down the line: If you were to plot the travel of the clubhead (or the hands) in three dimensional space that plot would be curved somewhat (depending upon the nature of the shifts) But the Plane is a flat Plane , which shifts angles. ( And maintains a straight line base line).
Its this straight line base line which is so ideal for tracing. The curve of the clubheads travel in three dimensional space is its geometric equivalent but an attempt to cover it would be far more difficult as your "curve" changes for any variation in length of club, ball position, plane shifts etc etc. The plane line just keeps on pointing straight line.
The circle lies flat (2D) on an inclined plane, which can shift its angle of inclination.
Words do a horrible job of describing geometry .......
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