"By rechak Kriya, sadhakas take the pranavayu residing above in the sushumna and offer it as oblation into the apanavayu residing at the bottom of the spine, and again by purak Kriya they lift the apanavayu from below and merge it into the pranavayu above. This has been mentioned in the yogashastras as "kevali pranayam."
By this, the movement of prana and apana stops naturally and on its own, stilling the mind and all prana, manifesting eternal happiness and illuminating the lamp of Knowledge. When the mind and prana are still in this way, the inhalation and exhalation of the physical airs, speech, body, sight - all of these things - also become still. One can ascertain the level of consciousness-absorption by these physical signs."
- Sri Yukteshvarji's Gita 4:29-31, English Translation Yoga Niketan
"One has to concentrate on the movement in the path of the Yogasutra-mentioned sushumna nadi to bring about that experience in the spinal column itself. Through this, that path continually gains magnetic power by the technique of Kriya, which results in turning the outgoing currents of the [other] nadis inward by the attractive power of that magnetic force, and [thus] the mind is also magnetically drawn within in that way. This results in the ending of the restlessness of inhalation and exhalation, after which, the state of pranayam where there is disconnection from the movement of in-breath and out-breath is eventually attained. This results in the stillness of mind, and with the disciplined practice of pratyahar and dharana -- dhyan and samadhi are eventually attained in the paravastha of Kriya. Through this practice itself one can attain realization on the path of samadhi, with the luminous experience of the various chakras - the abodes where power is transmitted - and the Unstruck Resonance of Pranava. It is because of this foundational and essential fact that Yogiraj has said that all states of sadhana can be attained with the reverently disciplined practice of the First Kriya."
- Swami Satyananda Giri from Kriyaa Quotes, English Translation Yoga Niketan