The power of Asana to bring about stability and foundation to the physical body is only the beginning of the action of Hatha Yoga. In order to achieve the higher results of the practice, the use of Pranayama, the controlling of the vital force and its flow in the body, is essential. Pranayama is frequently associated with control of the breathing, but this is just the most visible, and thus easiest to grab hold of, aspect of the control of the Prana, or vital life-force. Starting from a stable Asana, the practitioner will develop control over the breathing apparatus to first bring about a calm, alert control, and later to begin to direct and focus the flow of the vital force into different organ systems and for a variety of purposes.
Sri Aurobindo observes: “Pranayama, starting from the physical immobility and self-holding which is secured by Asana, deals more directly with the subtler vital parts, the nervous system. This is done by various regulations of the breathing, starting from equality of respiration and inspiration and extending to the most diverse rhythmic regulations of both with an interval of inholding of the breath. In the end the keeping in of the breath, which has first to be done with some effort, and even its cessation become as easy and seem as natural as the constant taking in and throwing out which is its normal action.”
It should be noted in passing that practicing Pranayama without guidance can be disruptive, as the practitioner is changing the automatic breathing function that is natural to the body. Without clear and precise guidelines the basic balance between mind, nervous system and physical frame can be disrupted and health issues can ensue. This may include physical health disruptions and mental balance issues.
That said, there can be real benefits to the proper implementation of the practices of Pranayama: “But the first objects of the Pranayama are to purify the nervous system, to circulate the life-energy through all the nerves without obstruction, disorder or regularity, and to acquire a complete control of its functionings, so that the mind and will of the soul inhabiting the body may be no longer subject to the body or life or their combined limitations. The power of these exercises of breathing to bring about a purified and unobstructed state of the nervous system is a known and well established fact of our physiology. it helps also to clear the physical system, but is not entirely effective at first on all its canals and openings; therefore the Hathayogin uses supplementary physical methods for clearing them out regularly of their accumulations. The combination of these with Asana,–particular Asanas have even an effect in destroying particular diseases,–and with Pranayama maintains perfectly the health of the body. But the principle gain is that by this purification the vital energy can be directed anywhere, to any part of the body and in any way or with any rhythm of its movement.”
Sri Aurobindo, The Synthesis of Yoga, Part Two: The Yoga of Integral Knowledge, Chapter 27, Hathayoga, pg. 512