What I see is that there are two mechanisms of support for the lower spine, one in the displacement of the fascia immediately behind the sacrum (by the mass of the extensors pressing rearward as they contract, usually in a natural alternation), and another in the displacement of the fascia behind the lower spine by pressure from the “fluid ball” of the abdomen (the term is from D. L. Bartilink). The fascia is the lumbodorsal fascia in both cases, but the coordination of these two means of support is effected by the autonomic nervous system as part of the movement of breath. Looks like this:
The bent-leg posture brings forward these relationships, yet the relinquishment of habitual activity in the movement of breath remains central to the experience of overall coordination.
The rotation of the mind at the dan tien is an experience of the coordination of the sense of equalibrium (vestibular organs) with the sense of proprioception (proprioceptors in the joints, muscles, and ligaments), the sense of gravity (otoliths), and the sense of vision (due to the tight connection between the sense of vision and the sense of equalibrium).
To unfurl the red flag of victory over your head, whirl the twin swords behind your ears; if not for a discriminating eye and a familiar hand, how could anyone be able to succeed?
(“The Blue Cliff Record”, trans. T. and J.C. Cleary, case 37 pg 274)
The experience of self-location through equalibrioception, informed by the “familiar hand” of proprioception and the “discriminating eye” that recognizes its role in self-location, can move. To the extent that equalibrioception is a part of the experience of self-location, there is pitch, yaw, and roll connected with the experience, and the coordination of sense in the experience of self-location can amount to a turning of “mind” (a turning of the experience of self-location) at the lower dan tien, on occasion.
I rely on the bent-leg postures and a natural inhalation and exhalation as my teacher.