The Hatha Yoga Pradipika states it without ambiguity: when Prana flows through Sushumna, the mind becomes still. When the mind becomes still, the self is recognised. This is the entire purpose of Hatha Yoga — and the Nadi system is the architecture through which it operates.
Ida Nadi — the left channel — begins at the left nostril, passes down the left side of the body, and curves to the base of the spine. It carries the lunar current — cooling, introspective, associated with the feminine principle, with the parasympathetic nervous system, with the right hemisphere of the brain. When Ida predominates: the mind turns inward, the body cools, the quality of consciousness is receptive and contemplative. Extended Ida dominance can produce heaviness, lethargy, over-introversion.
Pingala Nadi — the right channel — begins at the right nostril, passes down the right side, and curves to meet Ida at the base of the spine. It carries the solar current — heating, activating, associated with the masculine principle, with the sympathetic nervous system, with the left hemisphere of the brain. When Pingala predominates: the mind is active and outward-moving, the body heats, the quality of consciousness is executive and projecting. Extended Pingala dominance can produce agitation, over-activation, the burning that comes from sustained high-intensity output without restoration.
Ida and Pingala interweave at the seven major Chakra points as they ascend the spine — the pattern of the caduceus, the emblem of the healer in both East and West, whose deepest meaning is this dance of the two currents and the central channel they reveal.
Sushumna — the central channel — runs through the interior of the spinal column, from Muladhara at the base to Brahmarandhra at the crown. Within Sushumna, the texts describe three inner channels of increasing subtlety: Vajrini within Sushumna, Chitrini within Vajrini, and Brahma Nadi — the channel of Brahman — at the subtlest level. It is through Brahma Nadi that the Kundalini energy ascends at the highest levels of practice.
Kashmir Shaivism's treatment of Sushumna in the Tantraloka is explicitly cosmological. Sushumna is the axis mundi of the inner universe — the equivalent of Mount Meru in the cosmic geography, the central column around which all manifestation is organised. When Prana enters Sushumna, the individual cosmology aligns with the cosmic cosmology. The microcosm recognises its identity with the macrocosm. This is not metaphor — it is a precise description of what practitioners consistently report: the entry of Prana into Sushumna is recognisable as a specific quality of inner stillness and clarity that differs categorically from the states produced by Ida or Pingala dominance.
The practices that facilitate the entry of Prana into Sushumna: Nadi Shodhana pranayama — alternate nostril breathing that equalises Ida and Pingala. Kumbhaka — breath retention at the point of equilibrium. Moola Bandha, Uddiyana Bandha, Jalandhara Bandha — the energy locks that create the conditions for Prana to cease its horizontal oscillation between Ida and Pingala and move vertically through Sushumna.
The natural indicators that Prana has entered Sushumna: the breath becomes spontaneously equal through both nostrils simultaneously — a state that occurs naturally only at rare moments in ordinary life. The mind becomes genuinely still — not suppressed, not distracted, but intrinsically quiet. The quality of this stillness is recognisably different from ordinary relaxation. The practitioner who has experienced it once will recognise it again and will understand, at the level of direct experience, what all these texts have been pointing at.