[Yoda]

I'm not sure if you mean by the "head and the top of the spine" (the green text), the neck and the head (the cervical curve), or this section of the spine: somewhere in the thoracic curve, and from that point, to the head, this longer section of the spine remains fixed, motionless, having as an indicator: a fixed chest or shoulders, while the hips slide forward parallel to the plane line. At startdown.

Visualize the spine as the pendulum of a grandfather clock. It swings to and fro from a fixed center. That's the idea, and it is the way the golfer's Pivot operates (with respect to lateral motion). In accomplishing this objective, the Hips must move independently of the Head and Shoulders ('Hula Hula flexibility' per 7-14) in order for the Pivot to remain Centered.

Whether you choose to use the Head as the Pivot Center (Homer Kelley's recommendation as the First Essential / 2-0) or the point 'between the shoulders' (the alternative offered in The Glossary) is immaterial. Just keep something 'up top' stationary, and you will achieve your objective: Namely, a Centered Arc. It is no accident that the first two items in The Machine (1-L #1 and #2) deal with precisely this point.

By the way, stabilizing the lower part of the Pivot -- the Feet, Knees and Waist Bend -- goes a long way toward stabilizing the upper part. In fact, that is their major function (7-16).