The Pivot is a free Motion, but it nevertheless demands stability. That stability is provided by the Feet and Knees (at its base) and by the Pivot Center (fixed at its top). At the player's option, the Pivot Center can be either the Head or the 'point between the Shoulders.'
After careful consideration, Homer Kelley, author of The Golfing Machine, recommended the Head Pivot Center. Others prefer the 'point between the shoulders.'
With either Pivot Center, the top of the Pivot is stable. By definition.
Now let's think:
The spine connects the Head and Hips. The Hips move laterally -- this is the Weight Shift -- in both the Backstroke and the Downstroke. As the Hips move back and through, so does the spine. And because the top of the spine is attached to the Fixed Pivot Center, its bottom must move away from the Target in the Backstroke and towards the Target in the Downstoke.
It is strange that many who preach as gospel Downstroke 'Axis Tilt' fail to understand Backstroke Axis Tilt.