Through regular and diligent practice of concentration, yogis acquire various yogic powers, or siddhis, and become siddhas. This process is called sadhana. Pranayama is one of the most important sadhanas. Through the practice of asana, you gain control over your physical body, and through pranayama, over the subtle, astral body, or linga sharira. Since there is a very direct connection between the breath and the nerve currents, control of the breath means control of the inner life currents.