Pranayama is used to control, direct, and measure breath through various techniques. Because prana, the life force energy, rides the breath, pranayama has remarkable powers of transformation.
Many body functions are directed by the autonomic nervous system and operate without our control. Other physical actions are consciously directed by our thoughts and intentions. Breath is one function that is in between, straddling both the automatic and conscious systems. The body breathes perfectly well without any conscious direction, but we can also intervene and control our breathing. This gives us the potential to influence the life force energy within our bodies.
The Yoga Sutras consider pranayama, along with asana, to be the most effective means of purifying the body and mind. Both produce heat (tapas) the inner fire of purification. The subtle energy channels of the body, the nadis, are cleansed of obstructions to the vital force. This promotes healthy balance within the body and stimulates the higher centers of consciousness. Pranayama soothes and calms the mind. This reduces cravings.