Yoga Sutras of Patanjali and its eight limbs of yoga talk about pranayama as the fourth limb of yoga, after asanas and without which one cannot do dhyana, dharma or samadhi. Yoga Vashishtha, a dialogue between Vashishtha and Sri Ram Chandra talks in great detail about the importance of control of breathing for acquiring spiritual health.
In Allopathic language emotions are controlled by a balance in sympathetic and parasympathetic nervous systems. Anxiety increases sympathetic activity and calmness and stillness increases para sympathetic activity. The opposite is also true.
We all know that when we are tense the respiratory rate increases and during sleep the respiratory rate decreases. Therefore, to make the body and mind calm it we can use the technique of slowing down the respiratory rate.
In Vedic texts slower and deeper breathing is called pranayama and the process of putting a gap between inspiration or expiration is called kumbhakam. Bhastrika and kapal bhati pranayama are not true pranayamas as both involve increase in inspiration rates.
Pranayama, therefore, is a journey from a sympathetic to a parasympathetic mode. According to Yoga Sastras the life of a person is decided not by the number of years he or she lives but by the number of breaths he or she takes during the life span. Reducing the respiratory rate, therefore, increases longevity. By regular pranayama one can reduce the respiratory rate permanently. Pranayama can be done for a few minutes to begin with and can be increased to longer duration over a period of time. Once pranayama is learned the process of meditation becomes easier.
Pranayama can also be combined with various sounds like Om. These sounds create a resonance in the body and are known to vibrate autonomic plexuses and endocrine glands.
During pranayama, breathing can also be focused on various endocrine glands, Chakras or autonomic plexus. When combined with intent, this breathing is called advanced pranayama or siddhis. Pranayama gives various benefits known as siddhis in Vedic texts.
The basic breath awareness meditation, progressive muscular relaxation, shavasana, mind body relaxation, etc. are all extensions of various forms of pranayama. The suspension of breath after inspiration or expiration has been given importance in the Yoga Sutras of Patanjali. Practicing it helps going into the meditative phase early.
The concept of pranayama involves practicing breathing exercises over a longer period of time to increase the range, duration and rhythm of the breath so that it becomes deep and subtle.