I believe that its philosophy of finding a sitting posture is very much as Taigu and I encourage here at Treeleaf, namely, we each have to experiment with our own self and make small adjustments to find (within certain rules) the posture "right for my particular body" (one size does not fit all
). Further, sitting is not rigid and fixed, but always subtly fluid and changing, such that the posture at the start of a sitting period will not be precisely the same as at the end (or on different days!).
For this reason, the author presents a philosophy of sitting, and a series of exercises, to help each of us find our "sweet spot" (again, a "sweet spot" that is not stagnant, but needs to flow and change even during one sitting period). It is based on finding (1) an alignment of the body (head, neck, spine, buttocks, legs) that is balanced and in line with gravity (2) relaxed, yet (3) resilient. YOU KNOW IT WHEN YOU FEEL IT. Better said ... when the body feels right, and when the body feels balanced and "drops from mind" (becomes no longer a distraction), it probably is
right and balanced.