Yoga Sutra defines asana as that which is comfortable and easy, as well as firm. It is a dynamic position, in which the practitioner is perfectly poised between activity and non-activity, being doing and "being done by" the posture. A corresponding mental balance exists between movement and stillness. Yoga teaches that each posture reflects a mental attitude, whether that attitude be one of surrender, as in a forward bending asana, or the strengthening of the will, through backward bending postures, or the creation of a physical prayer or meditation with the body, as in the practice of padmasana (lotus posture). A posture or asana can be used for rejuvenating specific organs and glands as well as the spine.

There are about eighty-four asanas commonly used by yogis. We can, however, get sufficient benefits from a dozen of them. We will only present some of the more important ones here. If you what to learn more, there are many excellent books available that goes deeper into these asanas.

Many of the asanas have animal names, such as the fish posture and the cobra posture. This is because yogis devised their asanas partly by observing how animal instincts work in the wild. When animals are sick they would only eat certain herbs and grasses. Similarly, they would stretch and contract muscles in various postures instinctively.

Yogis also observed how animals relaxed. Cats, especially, are experts in relaxation. On awakening from sleep, they instinctively stretch, arch the spine in both directions and then relax.

Asanas are also based on a sound knowledge of human anatomy and physiology. Yogis knew that placing the body in certain positions would stimulate specific nerves, organs and glands. For example, the shoulder-stand posture causes the blood to be directed by gravity to the thyroid gland, and the tucking in of the chin causes a gently squeezing action on the gland. These two actions have a profound effect on the thyroid gland.

The asanas are based on five principles.

1. The use of gravity. The inverted postures such as the headstand, shoulder stand and the reverse posture take advantage of gravity to increase the flow of blood to the desired part of the body; in the headstand to the brain, in the shoulder stand to the thyroid gland and in the reverse posture to the gonads (sex glands)
2. Organ massage. The position of the asana causes a squeezing action on a specific organ or gland, resulting in the stimulation of that part of the body.
3. Stretching muscles and ligaments. This causes an increase in blood supply to the muscles and ligaments as well as relaxing them. It also takes pressure off nerves in the area.

This stretching is involved in all the asanas, since it has such a beneficial effect on the body.

4. Deep breathing. While holding the yoga posture we breathe slowly and deeply, moving the abdomen only (abdominal or low breathing). This increases the oxygen and prana supply to the target organ or gland, thereby enhancing the effect of the asana.
5. Concentration. As well as breathing slowly and deeply, we also focus our attention on the target organ or gland. This brings the mind into play, and greatly increases the circulation and prana supply to the organ or gland.

This concentration has the second benefit of increasing your general powers of concentration through regular practice. This benefits every aspect of your life. Your mind is less distracted and swayed by external events and you are therefore calmer and worry less. You will be able to solve day-to-day problems better and have more success in whatever activity you undertake.