Ardha means half. This is half a pose. This Asana takes its name from the Rishi or Yogi Matsyendra, who first taught this Asana to the students of Hatha Yoga. This Matsyendra is said to have been the disciple of Lord Siva. Once Siva departed for a lonely island. There He taught Parvati the mysteries of Yoga. A fish that happened to be near the shore heard the teachings of Lord Siva. Siva came to know this. His heart being filled with mercy He sprinkled water on this Yogi-fish. Immediately, on account of Siva's grace, the fish became a Siddha Yogi with a divine body. This Yogi fish was called by the name Matsyendra.
Paschimottanasana and Halasana bend the spine forwards. Dhanurasana, Bhujangasana and Salabhasana are counterposes to bend the spine backwards. This is not sufficient. It must be twisted and bent from side to side also (literal movements). Then only perfect elasticity of the spinal column can be ensured. Matsyendrasana serves this purpose well, in giving a lateral twist to the spinal column. Some Hatha Yogins start with this Asana when they give practical lessons to the Yogic students.
Place the left heel near the anus and below the scrotum. It can touch the perennial space. Do not allow the heel to move from this place. Perennium is the space between the anus and the external organ of generation. Bend the right knee and place the right ankle at the root of the left thigh and rest the right foot well on the ground, close to the left hip joint. Place the left Axilla arm-pit over the top of the vertically bent right knee. Push the knee now a little to the back so that it touches the back part of the Axilla. Catch hold of the left knee with the left palm. Then applying pressure at the left shoulder-joint, slowly twist the spine and turn to the extreme right. Turn the face also to the right as much as you can do. Bring it in a line with the right shoulder. Swing round the right arm behind the back. Catch hold of the left thigh with the right hand. Retain the pose from 5 to 15 seconds. Keep the vertebral column erect. Do not bend. Similarly, you can twist the spine to the left side. This will complete the spinal twist.
This Asana increases appetite by increasing the digestive fire. It destroys terrible diseases. It rouses Kundalini and makes the Chandranadi (moon) flow steadily. The moon is said to be located above the root of the palate. It drops the cool ambrosial nectar, which is wasted by mixing with gastric fire. But this Asana prevents it.
It keeps the spine elastic and gives a good massage to the abdominal organs. Lumbago and all sorts of muscular rheumatism of the back muscles are cured. The spinal nerve-roots and sympathetic system are toned. They draw a good supply of blood. This Asana is an auxiliary to Paschimottanasana.