Just as lions, elephants, and tigers are gradually controlled, so the prana is controlled through practice. Otherwise, the practitioner is destroyed.
– Hatha Yoga Pradipika, 2:15

The term vritti translates to "fluctuate," referring here to the subtle shifts in breathing patterns that can be consciously refined through practice. We find this practice in the 7th-8th century Mrgendra Tantra Yogapada and further details in Narayanakantha’s 10th century commentary on that text, though neither name it as a technique.

Vritti pranayama involves playing with the natural variations in our breathing, bringing greater awareness to the interplay between our inhalations, exhalations, and their transitions. In working with the rhythm and duration of our breathing, we can fine-tune it, and, with it, our awareness, emotional states, and energetic balance. It is also a vital tool for expanding our breathing capacity, gradually transforming what respiratory science refers to as our total lung capacity–n...