My only success in these matters has come as a result of the study of Western science, yet I will be the first to confess that what matters most to me is the relinquishment of habitual activity, conscious or unconscious, in the movement of breath.
With the accent on relaxation, freeing the direction of mind, and the experience of cessation, I do experience what Dogen’s teacher Rujing described as breath entering and reaching the dan-tien, breath exiting the dan-tien. Sometimes. What keeps it alive for me is this notion that Olaf Blanke has put forward, that our sense of self, especially the self that is associated with our sense of location, is a result of the coordnation of the vestibular and proprioceptive senses, along with the senses of vision and of gravity. Distinguishing these senses contributes to their coordination, as far as I can tell, and the lower dan-tien is really an experience of that sense of location in the surrender of activity in the movement of breath, behind (and usually below) the navel.
What’s it about. The three sets of abdominal muscles have ligamentous attachments to the rectus muscle, and these attachments are of equal length at a point a couple of inches below the navel.
Looking to be so relaxed, that the only activity in the body is activity generated by the ligaments themselves in response to stretch, and the movement of breath.
Two means of support for the lower spine, one in the fascia immediately behind the sacrum, where the mass of the extensors (as they contract in alternation) presses rearward against the lumbodorsal facial sheet, and the other from pressure generated by the abdominals in the “fluid ball” of the abdomen, also pressing rearward against the lumbodorsal fascial sheet but now behind the lower spine:
The two means of support are autonomically coordinated in the natural movement of breath.
The sense of self-location can occur at the lower dan-tien, a function of the individual senses Blanke described, as part of the autonomic coordination of displacement of the fascia behind the sacrum and lower spine in support of the movement of breath. The ilio-lumbar, ilio-sacral and abdominal ligaments generate activity in response to the weight of the body, the activity serves to align the spine to provide ease at appropriate nerve exits, the nerve exits allow feeling below the skin and throughout the body to guide support in the movement of breath.
I have details in a PDF, here (and a subsequent response more particular to reverse breathing, here).