Śivasaṃhitā 4.41

Mūla-bandha — ‘root lock’ — is here defined by its deepest result: vāyu entering suṣumnā, the central channel. It is not an end in itself but the catalyst for channeling prāṇa. The image of the heel pressing the anus is postural-technically specific — corresponding to siddhasana or muktāsana — not merely metaphorical.

Suṣumnā — literally ‘the very gracious’, from su- (very well) and ṣumna (grace, favor) — is the subtle spinal channel running through the interior of the vertebral axis. Madhyasaṅgata combines madhya (center, middle) with the participle of sam-gam- (to unite, to converge): the vāyu not only enters suṣumnā but settles in its innermost center, the brahmanāḍī. Abhyāsataḥ (through practice) is the causal instrumental marking the only gateway.

The epithet of Mūla-bandha as jarāmṛtyunāśaka — destroyer of old age (jarā) and death (mṛtyu) — is not rhetorical hyperbole but the expression of a physiological understanding: the descent of apāna is associated with catabolic processes (deterioration, elimination), while its reversal activates anabolic processes (regeneration, vitality). Regular practice of Mūla-bandha is considered in the tradition an agent of somatic rejuvenation (rasāyana).