Swara Yoga is the very ancient science that deals with the various qualities of the flow of the breath in both nostrils in terms of both cellular and cosmic import. The ancient Rishis believed that learning to read the breath and manipulate it, we can learn to read the outer and inner universe and also come into harmony and greater function.
The science is based on the observation that the breath has many subtle characteristics and in a healthy person "normally" will want to alternate
as being dominant from one nostril to the other approximately every 90 minutes.
There are many factors which influence this flow (swarodaya) including the
lunar cycles. Briefly, being conscious of what nostril is dominant gives us
information about the suitableness of certain activities. The left nostril dominance indicates ida dominance and the right nostril the pingala. The important characteristics of ida and pingala nadis are found in any good hatha yoga book so I won't detail them here. When the nostrils are balanced, the flow can go into the sushumna nadis. This is best for meditation and spiritual practices. These flows can be observed and manipulated any time during the day or even while sleeping (i.e., sleeping on the left side usually opens the right swara, and so forth). The swaras can also be opened through placing pressure on the opposite armpits and other methods some of which can be purely mental.
This is only the rudiments of Swara Yoga which is a large subject and is said to encompass the whole science of pranayama. What has been of great benefit to me and can be learned very easily is to balance the breath in both nostrils evenly before and during meditation (called sushumna breath).
Anyone can easily get in touch with which nostril is blocked by temporarily closing off the opposite nostril one at a time. The nostril which makes the highest pitch sound is the one that is most constricted and unless we have a deviated septum or other physical illness we can consciously learn to open up the clogged nostril simply through conscious intention (once our awareness is focused). When the breath is so balanced, then there is another shift in consciousness that directly relates deeply to the nervous system (cutting past the intellect) creating an ontological shift. At least this works consistently for me and I have been able to teach it to others. This sets a good basic frame for meditation as the mind is no longer discursive and if it does become agitated, returning to this even breath (called
sushumna breath) will again help eliminate the vrittis.
Books on swara yoga:
1) "Swara Chintamani: Divination by Breath", S. Kannan, Sagar Publications,
18, Indian Oil Bhawan, New Janpath Market, New Delhi, 1972.
This is a translation of the Swara Chintamani with appendices.
2) "Swara Yoga: The Tantric Science of Brain Breathing", Swami
Muktibodhananda Saraswati, Bihar School of Yoga, Munger, Bihar, India, 1983.
This is the translation of the Shiva Swarodaya with an extensive
introduction, analysis, and commentary.
3) "Secret Power of Tantrik Breathing", Swami Sivapriyananda, Abhinav
Publications, E-37 Hauz Khas. New Delhi -110016, 1996.
A small but excellent thorough explanation written with clarity.
4) Breath, Mind, and consciousness, Harish Johari, Destiny Books, One Park
St., Rochester, VT., 1989.
The most available book in the West and a reasonable and practical
introduction, but not an in depth presentation.
5) "Science of Yoga- Chapter Four -- Pranayama Section- Sub-Chapter on Swara
Yoga (pages 384-392, Swami Sivananda, The Divine Life Soc., India.