नाडी-शोधन, nāḍī-śodhana

Literally translated as "purification of nadis", channels of the subtle body. Alternative name – nadi-shuddhi (nāḍī-śuddhi).

Usually, nadi-shodhana means a certain pranayama – alternate breathing with a breath hold after inhalation and sometimes after exhalation. In yoga and tantra, it’s the most common way to clear canals.

Simple alternate breathing – anuloma-viloma  mainly affects Ida and Pingala, and in nadi-shodhana pranayama kumbhaka is added (first after the inhale and later on after the exhale), which "locks" or "seals" prana in the body, giving prana the way to transform and directs it into the central channel – sushumna.

Purification of the sushumna means that Kundalini-shakti can freely move in it and merge with Shiva. When Kundalini and Shiva "meet", spanda and stream of amrita emerge. One can say otherwise: when you hear (perceive) the vibration of "nada", it pulsates (spandana). This pulsation sometimes generates light and if you dissolve yourself, your perception in this experience, then the breathing stops on its own – this is kevala-kumbhaka.

Tantra often gives interesting explanations for the purification of nadis, for example, Kamalankar Mishra in one of his works says that the passage of Kundalini through the sushumna is the true nadi-shodhana. That is to say, when Shakti passes through sushumna and connects with Shiva in a state of supreme bliss, it is the purification of the nadis.

Nadi-shodhana pranayama is mentioned in "Hatha-Yoga Pradipika" (Chapter 2):

śuddhimeti yadā sarvaṁ nāḍīcakraṁ malākulam ।
śuddhimeti yadā sarvaṁ nāḍīcakraṁ malākulam ।
tadaiva jāyate yogī prāṇasaṁgrahaṇe kṣamaḥ ॥ 5॥

“When all the nadis and chakras which are full of impurities are purified, then the yogi is able to retain prana” (5)

prāṇaṁ vediḍayā piben niyamitaṁ bhūyonyathā recayet
pītvā piṅgalayā samīraṇamatho baddhvā tyajedvāmayā ।
sūryācandramasoranena vidhinābhyāsaṁ sadā tanvatāṁśuddhā nāḍigaṇā bhavanti yamināṁ māsatrayādūrdhvataḥ ॥ 10॥

Yogi should inhale prana through the left nostril, hold [the breath], then exhale through the other nostril. Then inhale through the right nostril, hold breath [inside] and then exhale through the left  [nostril]. By practicing [breathing] this way, through Surya and Chandra (right and left nostrils) alternately and following directions, [yogi] purifies all nadis in three months (10)

Hatha-yoga texts give different recommendations on when and how often to practice nadi-shodhana: some say four times a day, others suggest three or two times a day. This is due to such a key concept for Hindu as «sandhya» – the transitional time of the day when they make puja and pray. Since pranayama is an important element in the puja, and they use nadi-shuddhi in it, different recommendations on the frequency of execution may have developed. Tantrics follow four sandhyas, and some even five. But more often in Hinduism, three sandhyas are popular. Nathas mostly revere Gorakshanath, Guru (and possibly other deities) in their temples twice a day: at dawn and sunset, that is, when night crosses the day and vice versa.

Recommendations for practice

All hatha-yoga techniques must lead the practitioner to a dhyana state, and you can freely enter into this state with clean channels. The energy channels are often polluted by hovering in different sanskaras, vasanas, worldly feelings and emotions. Therefore, in order to purify the channels, you should not only perform pranayama as a breathing technique, but also try to make dhyana together with breathing, as well as adjust your lifestyle. 

Regularity of practice is important as well. If some types of pranayama can be performed from time to time, nadi-shodhana pranayama becomes effective only if practiced on the daily basis for 3 months or more. You can start at 15-30 minutes daily, in the morning and in the evening, gradually increasing the time. Depending on the way of life, sadhaka can spend on pranayama practice from 1 to 2 hours a day (socially active people) and up to 8 hours a day (sadhu, monks).

Consciousness flows in the nadis, that is why yoga texts say that the mantra is also important in pranayama, it purifies the mind. There are several basic mantras that are used in pranayama, one of them is OM (Pranava). Or it may be the mantra of Ishta-devata.

Nadi-shodhana technique

The practice should be started in the most calm, balanced way possible.

The breath can be started with the left nostril or with the one that breathes better at that moment. For example, close the right nostril, you should perform a smooth breath (inhale) through the left. Then hold your breath for a while. After a comfortable delay, open the right nostril, close the left, and exhale smoothly through the right nostril. And then in reverse order, inhale through the right nostril, hold your breath, exhale through the left nostril. 

At first it is better to perform nadi-shodhana in a natural rhythm, that is, with an arbitrary breath, exhalation and kumbhaka, and try to breathe as quietly as possible, so that the breathing itself gradually slows down and the mind calms.

Mantra can be added to pranayama, that is, mentally recite mantra when inhaling, then recite during the pause and then when you exhale. When a stable, serene state of consciousness is achieved, nadi-shodhana can be performed with certain proportions, and the mantra can help track these proportions. 

You should start at a ratio of 1:1:1 [inhale:hold breath:exhale]. After a while (a few weeks)  you can gradually start changing proportions, giving yourself enough time to master each option: 1:1:2, 1:2:2, 1:3:2. When breathing becomes very light, you can move to proportions of 1:4:2 and further 1:4:2:2 (i.e. add kumbhaka after exhalation).

You shouldn’t force longer breath holds or exhalations. Stretching time for kumbhaka or exhalation should occur very gradually (almost naturally) and not cause any discomfort. The transition from one proportion to another can take several months. It is not by chance that the texts say that taking control of prana is like domestication  of a wild beast:

yathā siṃho gajo vyāghro bhavedvaśyaḥ śanaiḥ śanaiḥ ।
tathaiva sevito vāyuranyathā hanti sādhakam ॥ 

"Just as lions, elephants and tigers are gradually controlled, so the prana is controlled through practice. Otherwise [practice] can destroy sadhaka." (2.15)