In plain words
It has sixteen letters. ‘A, ā, i, ī, u, ū, ṛ, ṝ, ḷ, ḹ, e, ai, o, au, aṃ, aḥ.’ It is made of sixteen petals joined together. We have to do it within the brāhma-muhūrta, from 3.30 to 5.30.
Then the Ājñā-cakra, which is at the place between the eyebrows. It is made of a part of just two petals. It has just the two letters ‘haṃ, kṣaṃ’. At the time of prāṇāyāma, in the sacred principle of the jīva, in the contemplation of the oneness of jīva and Brahman, we have to fix Manas on the place between the eyebrows alone, and through that perform the Haṃsa Gāyatrī. The breath we take in through this opening on the right is iḍā. In the teaching of prāṇāyāma this is what they have called pūraka. Letting it out is recaka. Recaka means letting out the air. When we understand the Haṃsa principle, the air let out goes as far as twelve feet. The place between this pūraka and recaka is kumbhaka. It is at the place between the eyebrows. That is, drawing in the air through the right opening is pūraka; then, for as long as we drew in that air, for so long we must hold it at the place between the eyebrows. That is kumbhaka. For as long as we held this air, the Haṃsa that is the principle of prāṇa, at the place between the eyebrows, for just that long we must let it go. In this, too, a science of number has been laid down. This is what they have also called Sāṅkhya yoga. For as long as we breathe in, for so long we hold it there at the place between the eyebrows, that is, kumbhaka; then for just that long we must let the breath out. The principle of the air breathed in we call the sun, and the principle of the air let out we call the moon. The moon, having no power of its own, depends on the sun for its light. In just the same way, recaka depends on pūraka. These two, pūraka and recaka, both depend on kumbhaka, that is, on the principle of the place between the eyebrows. We understand this only through our own experience; it does not become clear through teaching or through reading.