SB 3.28.9

prāṇasya śodhayen mārgaṁ
 pūra-kumbhaka-recakaiḥ
pratikūlena vā cittaṁ
 yathā sthiram acañcalam
Synonyms: 
prāṇasya — of vital air; śodhayet — one should clear; mārgam — the passage; pūra-kumbhaka-recakaiḥ — by inhaling, retaining and exhaling; pratikūlena — by reversing; — or; cittam — the mind; yathā — so that; sthiram — steady; acañcalam — free from disturbances.
Translation: 
The yogī should clear the passage of vital air by breathing in the following manner: first he should inhale very deeply, then hold the breath in, and finally exhale. Or, reversing the process, the yogi can first exhale, then hold the breath outside, and finally inhale. This is done so that the mind may become steady and free from external disturbances.
Purport: 

These breathing exercises are performed to control the mind and fix it on the Supreme Personality of Godhead. Sa vai manaḥ kṛṣṇa-padāravindayoḥ: the devotee Ambarīṣa Mahārāja fixed his mind on the lotus feet of Kṛṣṇa twenty-four hours a day. The process of Kṛṣṇa consciousness is to chant Hare Kṛṣṇa and to hear the sound attentively so that the mind is fixed upon the transcendental vibration of Kṛṣṇa’s name, which is nondifferent from Kṛṣṇa the personality. The real purpose of controlling the mind by the prescribed method of clearing the passage of the life air is achieved immediately if one fixes his mind directly on the lotus feet of Kṛṣṇa. The haṭha-yoga system, or breathing system, is especially recommended for those who are very absorbed in the concept of bodily existence, but one who can perform the simple process of chanting Hare Kṛṣṇa can fix the mind more easily.

Three different activities are recommended for clearing the passage of breath: pūraka, kumbhaka and recaka. Inhaling the breath is called pūraka, sustaining it within is called kumbhaka, and finally exhaling it is called recaka. These recommended processes can also be performed in the reverse order. After exhaling, one can keep the air outside for some time and then inhale. The nerves through which inhalation and exhalation are conducted are technically called iḍā and piṅgalā. The ultimate purpose of clearing the iḍā and piṅgalā passages is to divert the mind from material enjoyment. As stated in Bhagavad-gītā, one’s mind is his enemy, and one’s mind is also his friend; its position varies according to the different dealings of the living entity. If we divert our mind to thoughts of material enjoyment, then our mind becomes an enemy, and if we concentrate our mind on the lotus feet of Kṛṣṇa, then our mind is a friend. By the yoga system of pūraka, kumbhaka and recaka or by directly fixing the mind on the sound vibration of Kṛṣṇa or on the form of Kṛṣṇa, the same purpose is achieved. In Bhagavad-gītā it is said that one must practice the breathing exercise (abhyāsa-yoga-yuktena). By virtue of these processes of control, the mind cannot wander to external thoughts (cetasā nānya-gāminā). Thus one can fix his mind constantly on the Supreme Personality of Godhead and can attain (yāti) Him.

Practicing the yoga system of exercise and breath control is very difficult for a person in this age, and therefore Lord Caitanya recommended, kīrtanīyaḥ sadā hariḥ: one should always chant the holy name of the Supreme Lord, Kṛṣṇa, because Kṛṣṇa is the most suitable name of the Supreme Personality of Godhead. The name Kṛṣṇa and the Supreme Person Kṛṣṇa are nondifferent. Therefore, if one concentrates his mind on hearing and chanting Hare Kṛṣṇa, the same result is achieved.