I recently wrote a paper on different types of centralised backswings,and I coined the term "leftwards-centered backswing" and rightwards-centered backswing." I now think that there are golfers, like Johnny Miller, whose centralised backswing is intermediate between leftwards-centered and rightwards-centered and I will call it vertical-centered.
Consider a comparison between Mike Finney's backswing and Johnny Miller's backswing. Both golfers golfers have a centralised backswing - which I define as a torso pivot-rotation where the torso remains within the vertical limits of both inner feet. However, Mike Finney has a rightwards-centered backswing.
In this image, note that Mike Finney has a slight degree of rightwards spinal tilt at address. During the backswing, he keeps his head perfectly stationary and he ends up with a rightwards tilt of his upper torso and spine at the end-backswing position. Why does that occur?
This is my anatomical explanation. At address, Mike Finney has a slight rightwards spinal tilt due to tilting of the thoracic and cervical spine to the right. The lumbar spine cannot rotate or tilt (because of inbuilt anatomical constraints) so it has to remain centrally-oriented if the pelvis is horizontal and square to the ball-target line. The next diagram depicts Mike Finney's spine position at address (see rightwards-centered backswing) - his spine is slightly tilted to the right and his head is BEHIND the center of his stance.
Then consider what happens to the spine during the backswing pivot action.
When the pelvis pivots in the backswing, the right pelvis/buttocks moves rearwards and leftwards, and that causes the lumbar spine to become oriented to the right. Because the lumbar spine cannot rotate/tilt, the entire lumbar spine must acquire a rightwards tilt. Because Mike Finney's head has remained stationary during the backswing, his thoracic spine becomes straight-in-line with his lumbar spine, so that his ENTIRE spine appears STRAIGHT with a slight rightwards tilt (see diagram above).
I don't have a back-view of Mike Finneys' swing so I will use Nick Faldo as an example of a rightwards-centered backswing swing style golfer.
Note that Nick Faldo's ENTIRE spine is straight and that it has a slight rightwards tilt at the end-backswing position.
Now consider what happens to a golfer who has a vertical-centered backswing swing style. Let's presume that his spine has a very slight rightwards tilt at address, although the lumbar spine remains vertical (for reasons already given). During the backswing pivot action, the lumbar spine has to acquire a rightwards tilt (for reasons already given). However, the degree of rightwards tilt is going to be less in Johnny Miller's swing because he tilts his pelvis more during the backswing.
Note that Johnny Miller allows his right leg to straighten during the backswing and that the right thigh is consequently leftwards-inclined. Note that his pelvis is tilted (right pelvis-up and left pelvis-down). Note the buckling-in of the left knee due to the fact that the left pelvis is lower than the right pelvis. The effect of this pelvis tilt on the lumbar spine is to DECREASE the rightwards tilt that the lumabr spine naturally acquires during the backswing pivot action. That means that the lumbar spine will be near-vertical at the eend-backswing position. Then, when Johnny Miller rotates his shoulders >90 degrees during the backswing, his thoracic spine is torqued leftwards so that it is more vertically-aligned with the lumbar spine to create a spinal axis that is near-vertical. That accounts for the fact that the two red dots in the photo are vertically stacked-over each other at the end-backswing position.
What do you think of my anatomical explanation?
Jeff.