Why do the texts say that paścimottānāsana activates suṣumṇā and awakens the kuṇḍalinī?

It so happened in India, that spiritual practices are most often performed facing east. Accordingly, the west (conditionally) is our back, the south is the right and hot side and the north is left and cold. The Sun is associated with vital power, it rises in the east and sets (i.e. disappears) in the west. We fall asleep at sunset and our senses become silent, and they turn on at sunrise. We can see basically everything that is in front of us, on the sides, below and even sometimes from above, but we cannot see our back. We can only feel it, while there should be an inversion element, i.e. direction inward, listening to our sensations inside. The west is a symbol of the extinction of activities, the completeness of them, so it is suṣumṇā. It is also no coincidence that one of the well-known traditions, where kuṇḍalinī is worshiped in the form of the Goddess Kubjikā, is called paśchimāmnayā – the Western Doctrine. Paśchima – from paścāt (behind, the last, completed, western), therefore it is a symbol of suṣumṇā, self-absorption. Thus, paścimottānāsana is focused specifically on the direction of attention and the prāṇa in suṣumṇā along with it. This āsana is also focused on the conscious activation of suṣumṇā with pratyāhāra and the awakening of the kuṇḍalinī power.

Venerable Suṣumna

In general suṣumna is very glorifying in Haṭha-yoga-pradīpikā, many texts say that suṣumna contains the entire universe, it is very revered. The term itself comes from the root सुम्न (sumna), which means something desirable, magnificent with a strong prefix सु (su), where the “s” is transformed according to the sandhi rules into “ṣ”, thus the word suṣumna (very gorgeous) appeared. It comes up from the name that this channel is worthy of extremely serious perception, and Siddha-siddhānta-paddhati and a number of other texts describe it as a goal (lakṣya).

Sometimes there are translations of the term, like where the sun’s rays are. Obviously, this context comes from such early texts as Taittirīya Samhitā (3.4.7.1):

सुषुम्नः सूर्यरश्मिश्चन्द्रमा गन्धर्वस्तस्य नक्षत्राण्यप्सरसो वेकुरयः |
suṣumnaḥ sūryaraśmiścandramā gandharvastasya nakṣatrāṇyapsaraso vekurayaḥ |

Thanks to (suṣumna) moon, shining with the rays of the sun, is gāndharva, and his āpsaras (companions of gānharva) are the nakṣatras.

Further, it follows from the text, that this is described in the context of a fiery yajña, where the fire also relates to gānharva, the radiant rays of the flame – to āpsaras. In fact, the fire ofsuṣumna is the interiorised fire of the external yajña. Apparently, the term itself has Vedic origin.

Sometimes it is difficult to say where the object of reverence acts as a “support”, and where it is the goal.