I notice piriformis highlighted on that last set of illustrations.
For me, accumulation of chi in the tan-t’ien is a reference to the consciousness that occurs naturally in the vicinity of the tan-t’ien, and sometimes that consciousness almost seems like it’s continuously at the tan-t’ien but I don’t believe it is just that. It’s an inclusive thing, around the tan-t’ien. Reciprocal innervation of the piriformis muscles, from the femur to the sacrum on either side, results from the “hammocking” of the hips from the pelvis described in Calais-Germain’s “Anatomy in Movement”, which in turn stretches the ligaments associated with the piriformis muscles and generates piriformis activity. I think that activity also comes out of the stretch of the ilio-tuberous ligaments from the sacrum to the front sides of the pelvis, in the motion of yaw at the sacrum.
Anyway, the piriformis rotates the sacrum around the vertical axis, and I believe generates activity in the extensors up the back of the spine in three sets to the temporal bones and the parietals. The movement of the parietals affects the nerves that control the volume of cranial-sacral fluid in the skull and down the spine into the sacrum. The changes in the fluid volume cause flexion and extension of the sacrum, and of the sphenoid bone in the skull, in the middle of which sits the pineal gland.
But as to how consciousness or the heart-mind occurs naturally in the vicinity of the tan-tien, that is falling down while waking up or falling asleep, and never hitting the ground; the action of being upright is generated in the stretches engendered in falling down, directly. Does awareness at the tan-t’ien seem more natural and come more often to me now in the lotus? Yes, but storing this depends on the natural attractiveness of a state of absorption in the body, not on something I do. Has to be something I just can’t help because it is my own wellness that I’m feeling, no?