Thousands of years after Matsya the fish received the teachings of yoga from Shiva, he was reincarnated as Matsyendra (sometimes also referred to as Matsyendranath), which means Lord of the Fishes (matsya = fish, indra = lord, nath = refers to Nath group).
Matsyendra was a Nath Warrior, who was also a half man, half fish (a mer-man!) The Nath Warriors were a power-hungry group of people during the 10th-14th centuries who would do whatever it took to become more powerful – pillaging villages, tearing down forests, destroying everything around them without giving a second thought to the destruction they caused.
Matsyendra had heard about this strange group of people who hung out in caves deep in the woods meditating and contorting their bodies in all sorts of ways – and they also had superpowers. These yogis, as they were called, could predict the future, become invisible, levitate, had super-strength, could go a long time without food or drink, had super-sonic hearing, and so much more. The Nath Warriors thought that if they had all of these powers, they would be invincible and could take over the world! So they took a break from destroying the world to study yoga. Eventually they transformed and realized there was much more to life than taking over the world. Through long and consistent practice (abhyāsa) but also non-attachments to the results of yoga (vairāgyā) they found happiness, joy and bliss, and became the Nath Yogis, rather than the Nath Warriors.
Matsyendra shared these physical yogic practices with Swami Svatmarama, who wrote them down, and they were passed down to us in the form of the Hatha Yoga Pradīpikā. Each chapter of the HYP starts off with an invocation to the Lord Shiva who was the first to pass down the teachings of yoga.
In an āsana class we take the form of Ardha Matsyendrāsana (sometimes referred to as seated spinal twist) as an homage to the lineage of teachers who passed down the teachings of yoga either directly or indirectly to us. The upper body represents the torso of a man, while the folded legs resemble the tail of a fish. This āsana is not only a twist, but also an outer hip opener (on the side you are twisting). It promotes spinal health and flexibility, and also stimulates our digestive function. We typically twist to the right first to allow food to move up the ascending colon, and then to the left down the descending colon.