In this asana posture the shape of the body appears similar to fish in the water. Such a statement is bold as the fact is we don't recognize the posture as fish but one can float on the water without any movement in this asana.
Jalandhar Bandha in Sarvangasana pressurizes the thyroids and pituitary glands but in Matsya Bandha these glands are stretched. These opposite processes help improve the circulation in these glands assuring healthy functioning. Also the thigh & abdomen muscles are stretched systematically. Matsyasana is a lying (supine) pose, as well as a backbend.
It is one of the more active of the supine group, although the weight of the body is supported by the legs, arms and (to a lesser extent) by the head. In some respects it has more in common with backbends such as setu bandhasana or a gentle ustrasana, particularly for beginning students