Definition via KPJAYI.org:
This means the three places of attention or action: posture, breathing system and looking place. These three are very important for yoga practice, and cover three levels of purification: the body, nervous system and mind. They are always performed in conjunction with each other.
Asanas [āsana] purify, strengthen and give flexibility to the body. Breathing is rechaka and puraka, that means inhale and exhale. Both the inhale and exhale should be steady and even, the length of the inhale should be the same length as the exhale. Breathing in this manner purifies the nervous system. Dristhi [dṛṣṭi] is the place where you look while in the asana. There are nine dristhis: the nose, between the eyebrows, navel, thumb, hands, feet, up, right side and left side. Dristhi purifies and stabilizes the functioning of the mind.
For cleaning the body internally two factors are necessary, air and fire. The place of fire in our bodies is four inches below the navel. This is the standing place of our life force. In order for fire to burn, air is necessary, hence the necessity of the breath. If you stoke a fire with a blower, evenness is required so that the flame is not smothered out, or blown out of control.
The same method stands for the breath. Long even breaths will strengthen our internal fire, increasing heat in the body which in turn heats the blood for physical purification, and burns away impurities in the nervous system as well. Long even breathing increases the internal fire and strengthens the nervous system in a controlled manner and at an even pace. When this fire is strengthened, our digestion, health and life span all increase.
Uneven inhalation and exhalation, or breathing too rapidly, will imbalance the beating of the heart, throwing off both the physical body and autonomic nervous system.
An important component of the breathing system is mula and uddiyana bandha. These are the anal and lower abdominal locks which seal in energy, give lightness, strength and health to the body, and help to build a strong internal fire. Without bandhas, breathing will not be correct, and the asanas will give no benefit. When mula bandha is perfect, mind control is automatic.