Matsyendrasana is the series’ twist of the spine. In this asana, we concentrate on extending upward, creating space between the vertebrae so that we can use that space to twist further. As with all asanas that elongate and contort the spine, energy is released along the spinal column, creating heat and energy in the body.
Sivananda Companion to Yoga says of the spinal twist:
In this pose every vertebra is rotated on both sides. As in many of the sitting asanas, you use both the floor and parts of your body as levers – pulling on your ankle, pushing the backs of your arms against your knees to increase the stretch. As you twist your torso around, imagine you are wringing out a wet cloth. This squeezing movement stimulates circulation to the spine and prevents stagnation in the internal organs, riding the body of toxins and breaking down fatty tissue. Prana [energy] suffuses the entire spinal area, giving you added strength and concentration.
The spinal twist is followed by a standing forward bend and then the final posture, the triangle.