...doing Kriya -- cold air (vayu) moves within and outside the spine.
Eternal kootasth (vibration) - doing kriya the cool air goes in and out of the meru.
[Comment: Meru is defined at the end of the commentary on this verse as “spinal cord”]
Making vayu like a rope from the throat to the eyebrows - “kua” [well] - meaning throat, heart, navel, lingamula, Muladhar - all the lotuses that are there in these five elements - draw vayu through the roadway of this tube, bring it into the middle of the eyebrows - meaning: into the Ajna chakra and merge there.
Swami Sri Yukteshvar:
Under the brain and above the spinal column, there is a nadi (medulla) located between the eyebrows formed like the shape of the back of a tortoise. When one has total absorption - samyama - in that tortoise-nadi, then immediately restlessness is eradicated and a tranquil Light prevails. About this subject, here is what is written in the Patanjal philosophical treatises: as in -
kurmanadyam sthairyam - Bibhutipada - no. 32
It is signified by being two-petaled (pons varoli). This the Ajna Chakra. Through this, the mind enters the sushumna. This is the door of the sushumna. As in -
dhyanatma sadhakendra bhavatiparpure ityadi - Shatachakra - no. 35
Like a string, that tortoise-nadi extends through the center of the spinal column to the area of the anus. That is called “sushumna” (spinal cord).