Matsyasana is one of the postures the Gheranda Samhita builds out of another. The practitioner takes the bound lotus, holds it, lies back onto the ground and supports the head with the elbows. The text claims that the posture destroys disease, a claim the atlas records as the text's own.
Only the later manual carries it. The Hatha Yoga Pradipika's list stops well short of the reclining forms, and matsyasana belongs with the crocodile, the locust and the serpent to the material that widened the repertoire between the two compilations. It is also one of a small set, with the stretched tortoise and the stretched frog, in which a posture is defined by reference to the one it comes out of, so that the entry only makes sense to a reader who has the earlier entry in hand.
Matsya is the fish, and the surviving descriptions do not say what about the arrangement is fish like. The manuals routinely leave their images unexplained, and where a text is silent the atlas records the silence rather than supplying a resemblance of its own. What is clear is that the posture sits inside the lotus family: the legs stay in padmasana throughout, so matsyasana in the classical description is unavailable to a practitioner who cannot take the lotus, which is not true of the shape usually taught under the name today.