Pranayama is the 4th limb of ashtanga, the system of yoga the sage Patanjali described in his Yoga Sutra. It means breath control or extension of life energy. Through specific breathing exercises, one can learn to regulate the flow of energy through the body creating harmony.
Once harmony with the physical body has been achieved (asana), through interruption of the movement engendered by inhaling and exhaling you attempt to harmonize your energy (pranayama). Exhalation, inhalation, retention, technique, time and number must be very precisely regulated over a lengthy period. The fourth pranayama technique ultimately transcends breath retention after exhaling or inhaling. The veil covering the light of the true self then vanishes. – Yoga Sutra 2.49-52
Breathing is an automatic process, yet it is absolutely necessary for life. The way we breathe affects everything – how we think, feel and behave. When unexamined, the breath tends to take on habitual patterns that can contribute to suffering. For example, when we are frightened or stressed, breathing may become shallow or ragged. When we over exert ourselves, we may hold the breath or unevenly gulp in great quantities or air without exhaling fully.
Practicing pranayama is a method of becoming more familiar with breathing patterns and learning to influence them. By becoming more conscious of breathing, we may notice when the breath is constricted or unbalanced and make changes as appropriate. The practice of pranayama is a very rich subject with a broad range of exercises, so I will stick to a more familiar and simplified Western style explanation of how it can be useful.
The autonomic nervous system (ANS) is a largely unconscious physiological system that controls the internal organs and regulates associated bodily functions. The sympathetic nervous system is a division of the ANS that quickly activates the fight or flight response, while the parasympathetic nervous system (PSNS) is the division responsible for the rest and digest response, gradually returning the body to homeostasis. The PSNS conserves energy through slowing respiration and relaxing certain muscles.
Through pranayama, we gain access to these systems. We can use pranayama to train ourselves to regulate heart rate and blood pressure, and to heat, cool or relax the body, bringing ourselves into greater balance. The practices allow us to influence functions that are typically automatic. In this way, we can use the breath as a vehicle driving attention to typically unconscious aspects of being. This state of energetic equilibrium prepares us for subsequent stages of ashtanga, including concentration and meditation. You can learn more about science’s recognition of the health benefits of pranayama in the New York Times’ recent article on this subject.
Still others offer as sacrifice the outgoing breath in the incoming breath, while some offer the incoming breath into the outgoing breath. Some arduously practice prāṇāyām and restrain the incoming and outgoing breaths, purely absorbed in the regulation of the life-energy. Yet others curtail their food intake and offer the breath into the life-energy as sacrifice. All these knowers of sacrifice are cleansed of their impurities as a result of such performances. – Baghavad Gita 4.29-30