The Axis Mundi of the yogic body, the Sushumna Nadi -- known in Taoist parlance as the Chong Mai or Penetrating Vessel -- is arguably the most important of the Eight Extraordinary Meridians, which together represent the human body’s deepest, most primordial level of energetic functioning.
In relation to the physical form, the Sushumna / Chong flows slightly anterior and parallel to the spinal column, from the pelvic floor (Hui Yin/REN1) to the crown of the head (Bai Hui/DU20). [Note: in certain Chinese Medical contexts, the Chong’s trajectory is described differently.]
In relation to the energetic matrix within which the physical form appears, the Sushumna / Chong is the central portion of a torus-shaped energy-body, which encapsulates the physical in a way similar to how an egg contains a chick, or a womb contains an embryo. The fountain-like flow of energy up the central channel of the Sushumna expands at the crown of the head (like a “thousand-petaled lotus”) -- before cascading down (in a 360-degree arc) to join once again at the root of the channel (the pelvic floor), where the cycle continues.
Whereas the twelve main meridians are conduits for life-force energy (qi/chi) associated with our everyday dualistic space/time experience, the Sushumna Nadi carries the nondual energy of Primordial Consciousness: a more refined, purified and primordial (or “prenatal”) form of qi.
In the Tibetan yogic tradition, the light or energy that flows through the central channel is known as “wisdom air” -- and is believed to correspond to the very subtle mind (which transcends space/time, i.e. is nondual & nonconceptual).
As I’ve written elsewhere:
“The third level of mind is known as the "very subtle level" or the extraordinary level -- which exists ontologically prior to the space/time matrix of our ordinary (coarse & subtle) mental continuum. It is within this very subtle level that abide, and can (by a skilled meditator) be perceived, various karmic propensities/probabilities or "seeds" -- the deepest level of "transpersonal" conditioning -- along with the "birthing" of these karmic seeds into a particular space/time expression.”
(Some have suggested this “very subtle mind” -- and the corresponding “energy” of the Sushumna nadi -- to be an energetic correlate to what in western medicine/science is known as our “genetic imprinting,” i.e. that which somehow transcends the appearance of a bodymind’s single lifetime.)
In relation to Taoist yoga practice (qigong, taiji etc.), it’s useful to conceive of the Sushumna Nadi / Chong Mai as a portal, more than as a destination. In other words, allowing awareness to flow within the so-called “central channel” can be a means of transcending dualistic identification of all sorts, including (fundamentally!) the identification with a specific physical body, with a specific space/time location. But this “works” only to the extent that, at some point -- after it has served its purpose -- we are able to dissolve the visualized location of the “central channel” -- in relation to our physical form. Otherwise, the Sushumana Nadi can become simply a subtle reification of our notion of ourselves as being a separate, permanent “form” (albeit a subtle one).
As Thinley Norbu, in White Sail, writes:
"Wisdom air has no limitation of substantial form, but because practitioners have karmic air and the habit of form, the central channel is generally visualized as the size of the shaft of an arrow."
So: we initially visualize the “central channel” in relation to our (also-visualized in the sense of “projected”) physical form. Given the structure of the human subtle body, and its relationship to the physical nervous system, this somehow functions as a skillful means, to awaken nondual awareness, i.e. a felt awareness as ourselves as Tao.
Once the visualization has served this purpose, the practice then finds its completion in the dissolving of the visualized “central channel” along with the “physical form” that served as its initial point of reference. This leaves us with the entire (also-projected) “universe” as our “body” -- all appearances experienced as arising within who-we-are, essentially, rather than being divided into “self” and “other” in relation to a physical form conceived of as being “mine.”
And then, of course, separation (into apparent “self” and “others”) reappears -- but in a more-transparent, playful way: as the ever-transforming dance of yin and yang, this and that .... simply for the sake of play, of a kind of cosmic theater, not to be taken all that seriously ....