[KevCarter]

I am comfortable with HK's view. This should be easy geometry. The component- Shoulder turn, The variations- HK illustrates 5,
lets reduce it to 3. We place a disk centered at the golfers axis of rotation. put a point on the circumferance of the disk.

#1 the axis and disk are not rotated- Varation ZERO
#2 The disk and axis are rotated such that the disk plane is always perpendicular to the axis of rotation- variation ROTATED
#3 The disk plane is not perpendicular to the axis and therefore the disk will appear to "wobble" on the axis as the disk and axis are rotated. This is the variation that exists for "flat" backstroke and "on plane" downstroke or other planes that dont meet #1 or #2.
There is more geometry. If, the DS plane Top is not precisely located for a proper "wobble" the golfer can not obtain/maintain a straight plane line without compensations. This "wobble is set BOTH by amount of backstroke rotation and axis tilt at/near the top.

Just my opinion-All disclaimers apply

Ps. Which leads to the question - Should the shoulder be on a downstroke plane to the plane line or to support the hands?

HB

Interesting analysis.