Matsyasana means: “Fish posture”
Ardha Matsyendrasana translates as: “Half-Fish-Lord pose”
Purna Matsyendrasana is then the “Full-Fish-Lord pose”
The “Fish” in all three of these postures is referring to the first avatar of Lord Vishnu. This myth first appears in the Shatapatha Brahmana (700-300 BCE). This story is one that we might be more familiar with than we realize...
It takes place at the end of an epoch, when the quality of virtuous people inhabiting the planet has decreased to an all-time low. Lord Vishnu, in his compassionate mercy, takes the form as a fish to save the world from spiritual darkness and complete destruction.
Satyavrata (who later become Manu - the first human) is out walking one day, and comes across a tiny fish looking up at him from a deep pond. When the fish begins speaking to him, Satyavrata realizes this is no ordinary fish. He scoops him up and takes him home inside a jug of water.
Matsya keeps growing and growing, and Satyavrata continues moving him from one jar into another, always needing larger and larger containers, until he has no choice but to move him into the ocean.
This is possibly where we get the progressions of these postures. Each referring to a different period of Vishnu's transformation from a simple little fish, to the size of a 'half-king-fish' and finally into the form of a 'full-King' fish!
At this point, when the Ocean is the only body of water large enough to hold Matsya, the sage recognizes the fish to be the incarnation of the Lord of the Universe, and it is only then, upon recognition, that Matsyendra warns Satyavrata about the coming dissolution of the world. He instructs him to build a boat, to fill it will all the animals two by two, also he is told to bring different varieties of plants and grains, as well, not to forget the sapta-rishis (Seven Sages) - they should of course come too.
It is interesting here to mention that the name “Satyavrata” means ‘Truth-Vow’ or ‘True-Devotion,’ and we can assume that he was chosen (like Noah) because he was truly a virtuous man during a time when there was copious amounts of adharma (immorality, wickedness and vice).
At some point during the flood, a terrible storm arises and the boat is about to capsize. Matsya swims up to save the ship. Manu ties a rope around the giant fish and the fish-god guides the boat safely into the Himalayas, above the tumultuous tides.
Lastly, the fish bestows upon Manu the gift to recreate the earth and Satyavrata finally becomes Manu. The Sanskrit term for 'human' is manava, which literally translates as “of Manu” or ‘Children of Manu”.
It is interesting to observe that every culture seems to have its own version of the “great flood” story. All of these accounts have a common theme in that they depict a flood, usually sent by a god or gods as an act of Divine Retribution, to destroy or cleanse a civilization that has become evil and are no longer living in accordance with god’s virtuous ways.
These stories were told to perhaps serve as a moral warning against a tendency towards corruption. They explain how forces beyond our own understanding can destroy or come against people if they are too deviant, and give hope to those who believe themselves to be upright and noble.
Practicing Matsyasana or Matsyendrasana can help remind us that even the smallest minnow or largest shark holds within its very flesh the Supreme Consciousness that connects us all.
The practice of Ashtanga Yoga should act like an "inner flood" that purges us of our negative qualities and harmful tendencies. Through the daily ritual of practice we are asked to "yoke" or harness our minds to "The Lord" and surrender our struggles long enough to allow this Divine Absolute Consciousness to lead and guide us to higher ground.
Ultimately, it doesn’t matter what posture we make with our bodies; if the mind and spirit are not aligned with fair, just, kind, and compassionate qualities, than the point of the yoga practice is missing the mark... and we are left to simply wait for the flood.