Chapter IV Sloka 29

Apane juit a[< a[e=pan< twapre,
a[apangtI ωva a[ayampray[a>

apne juhvati pra 
pre'pna tathpare |
prpnagat ruddhv 
prymaparya ( 4|29)||

Others offer as sacrifice the outgoing breath in the incoming, and the incoming in the outgoing, restraining the course of the outgoing and the incoming breaths, solely absorbed in the restraint of the breath.
Commentary:

Some Yogis practise Puraka (inhalation), some Yogis practise Rechaka (exhalation), and some Yogis practise Kumbhaka (retention of breath).
The five sub-Pranas and the other Pranas are merged in the chief Prana (Mukhya-Prana) by the practice of Pranayama. When the Prana is controlled, the mind also stops its wanderings and becomes steady; the senses are also thinned out and merged in the Prana. It is through the vibration of Prana that the activities of the mind and the senses are kept up. If the Prana is controlled, the mind, the intellect and the senses cease to function.

Chinmayananda adds: There is a general misunderstanding that Prana is breath. This term is used in the scriptures to indicate the barious "manifested activities in a living body." The five different kinds of pranas, when understood correctly, are nothing but the different functions in every living body: 1) perception 2) excretion 3) digestion and assimilation 4) the circulatory system that distributes the food to all parts of the body 5) capacity to improve mental outlook and intellectual life. These are unconscious activities that are brought under control through pranayama so that the seeker gain capacity to withdraw all his perceptions which is a great help for the meditator.