The pain in the left knee, I’m pretty sure is related to the support or lack thereof for the fifth and fourth lumbar vertebrae in inhalation and exhalation.
Cheng Man-Ch’ing prescribed relaxation from the shoulders to the finger tips, from the groin to the soles of the feet, and from the sacrum to the crown of the head, followed by relaxation of the chest, as a means to sink the ch’i to the tan-t’ien. His advice implies a relaxed extension up the spine, both in inhalation and in exhalation. Particularly in exhalation, the alignment of relaxed extension becomes a function of the ability to feel right to the surface of the skin: here’s a dermatone chart that shows the relationship between feeling in the legs and nerve exits at specific vertebrae that allow the ability to feel:
I’m fascinated to see that at the tan-t’ien, it’s T10-12 in front, and L1-3 in back, so Gautama’s advice about “(conscious) behind as before, before as behind” would juxtapose feeling related to the alignment of the bottom-most chest vertebrae with feeling related to the alignment of the upper part of the lower spine.
“The ch’i can only sink to the tan-t’ien if the chest is relaxed”, as Cheng Man-Ch’ing said, and yet this statement doesn’t directly address the hypnogogic nature of the phenomena and how the necessity of breath is involved in that.
“The true man, breathing to his heels”- this is an old Chinese adage which concerns the ability to feel at the soles of the feet informing support at S1-L5 as a matter of necessity in the relaxed movement of breath.
The old man, Sasaki, knows all this intimately. Teaching it may distract people from their own sense of location, their own sense of gravity and proprioception, and so at Cobb Mountain last year he started his lecture with: “Good morning- where am I?”