The Gita says the exhaled breath is offered into the inhaled breath. And this is Kriya I believe. So my question is using our body when we inhale we are breathing out in the Astral spine and when we exhale using our body, we breath in through the astral spine, which means we are linking the astral and the material body breathing through Kriya. Which means physical breath is linked to the astral breathing and synchronises the cold and hot air prana flowing through astral spine. Jai Guru.
—Rajeev Jayaram, India
Paramhansa Yogananda taught that the Gita verse you cite above is indeed a reference, though not a literal one, to the technique of Kriya Yoga. But putting aside that technique so far as its details are concerned (which is not appropriate for this correspondence), the interpretation of the astral breath that you describe in your note is not what Yoganandaji taught.
This is what he taught: that at the time a newborn child takes his first breath, the flow of prana in the deep astral spine (the sushumna), moves out from the base of the astral spine into the the subsidiary channel known as the ira nadi. It runs upward along the left side of the sushumna (it actually intertwines the sushumna through the chakras as it moves upward to the brain). As it rises, it triggers the inhalation of air into the lungs. When the pran current reverses its flow becoming the apan current, it runs down along the right side of the sushumna (intertwining through the chakras) and causes the exhalation of air from the lungs.
Only at death or, for an advanced yogi, when the breath temporarily ceases during meditation, does this outward flow in the ira and pingala nadis (channels) cease. At such time the energy (prana) returns to the central, deep astral spine (sushumna). It rises up the sushumna and, whether at death or in leaving the body temporarily, exits through the medulla oblongata from which it came at time of conception.
The reflected light of prana entering (and exiting) the (astral) medulla is the light of the three concentric circles of the spiritual eye (kutastha).
I hope this is understandable. Better than mere intellectual understanding is the “getting” of actual, inner experience. Learn the technique of Kriya Yoga from a qualified teacher and you will be on the way to Self-realization of these universal aspects of our soul’s imprisonment in the body and its final release into the Infinite bliss of God.
Joy to you!