Pranayama is regarded lengthy or subtle according to its three components, the external, the internal and the steady; the retention processes are modified by the regulations of space, time and number.
When the breath is expired, it is Rechaka, the first kind of Pranayama. When the breath is drawn in, it is the second, termed Puraka. When it is suspended, it is the third kind, called Kumbhaka. Kumbhaka is retention of breath. Kumbhaka increases the period of life. It augments the inner spiritual force, vigour and vitality. If you retain the breath for one minute, this one minute is added to your span of life. Each of these motions in Pranayama, Rechaka, Puraka and Kumb-haka, is regulated by space, time and number.
By space is meant the inside or outside of the body and the particular length or the breadth and also when the Prana is held in some particular part of the body. During expiration the distance to which breath is thrown outside varies in different individuals. The distance varies during inspiration also. The length of the breath varies in accordance with the pervading Tattva (Element): Prithivi, Apas, Tejas, Vayu or Akasha (earth, water, fire, air or ether). This is again exter-nal during exhalation and internal during inhalation.
Time is, the time of duration of each of these, which is generally counted by Matra, which corresponds to one second. Matra means a measure. By time is also meant how long the Prana should be fixed in a particular centre or part.
Number refers to the number of times the Pranayama is performed. The Yogic student should slowly take the number of Pranayamas to eighty at one sitting. He should have four sittings in the morning, afternoon, evening and midnight, or at 9 a.m., and should have thus 320 Pranayamas in all. The effect or fruit of Pranayama is Udghata or awakening of the sleeping Kundalini. The chief aim of Pranayama is to unite the Prana with the Apana and take the united Pranayama slowly upwards towards the head.
Bahya-abhyantar-stambha-vritti-desaa-kala Sankhyabhih patidtishto deergha-sukshmah.
Yoga Sutras—Chap. II, Sa. 50