II.49 tasmin sati svasa-prasvasayor gati-vicchedah pranayamah
When that [asana] is accomplished, pranayama, breath control, [follows]. This consists of the regulation of the incoming and outgoing breath.
“Pranayama as breath control is an ancient practice that can be found in the old Brahmana texts. Vyasa explains that the svasa from this sutra is the intake of air from the outside, and pravasa, the exhalation of air from the stomach. He defines pranayama to be the suspension, or absence, of both—in other words, the suspension of breath.” The Yoga Sutras of Patanjali, Edwin F. Bryant
“It is difficult to explain Prana, as it is to explain God. Prana is the energy permeating the universe at all levels. It is physical, mental, intellectual, sexual, spiritual, and cosmic energy. All vibrating energies are prana. All physical energies such as heat, light, gravity, magnetism, and electricity are also prana. It is hidden or potential energy in all beings, released to the fullest extent in times of danger. It is the prime mover of all activity. It is energy which creates, protects, and destroys. Vigour, power, vitality, life and spirit are all forms of prana.”
“ Prana means breath, respiration, life, vitality, energy or strength. When used in the plural, it denotes certain vital breaths or currents of energy (prana-vayus). ‘ayama’ means stretch, extension, expansion, length, breadth, regulation, prolongation, restraint or control. ‘Pranayama’ thus means the prolongation of breath and its restraint.”
“Pranayama is an art and has techniques to make the respiratory organ to move and expand intentionally, rhythmically and intensively. It consists of long, sustained subtle flow of inhalation (puruka), exhalation (rechaka) and retention of breath (kumbakha). Puruka stimulates the system; rechaka throws out vitiated air and toxins; kumbakha distributes the energy throughout the body. The movements include horizontal expansion (dairghya), vertical accession (aroha) and circumferential extension (visalata) of the lungs and the rib cage.”
“The disciplined breathing helps the mind to concentrate and enables the sadhaka to attain robust health and longevity.”
“All stages of [Ujjayi Pranayama (Expanding Conquest Breath control)] except those with retentions may be done at any time. However, if the heart feels heavy, full or painful, or the diaphragm is hard, and if you are agitated or the heart-beat is abnormal, lie down…”
Light on Pranayama, B.K.S. Iyengar
II.50 bahyabhyantara-stamba-vrttih desa-kala-sankhyabhih paridrsto dirgha-suksmah
[Pranayama] manifests as external, internal, and restrained movements [of breath]. These are drawn out and subtle in accordance to place, time, and number.
“Moving on to the second part of the sutra, all these different types of breath restraint are regulated by place, desa, that is, the surface area that is reached by the breath, says Vyasa. He understands time as the seconds of duration of these cessations of the flow of breath, and number as how many sequences of inhalations and exhalations are restrained, and whether they are mid, middling, or intense in nature.”
“Time, kala, refers to differing durations of each individual exhalation, inhalation, and retention,… Number, sankhya, is the number of reputations, or rounds of each cycle of inhalations, exhalations, and retentions at one sitting.”
“In other words, say the commentators, one can increase the duration of these intervals of breath restraint so that they become more and more prolonged and imperceptible in terms of the moment of air. The Yoga Sutras of Patanjali, Edwin F. Bryant
Rafael Durán 