In the article "Heroes, Saints, and Sages" at yogajournal.com, Colleen Morton Busch writes:
"Matsyendra appears to have been an actual historical person, not just a figure of myth. Born in Bengal around the 10th century c.e., he is venerated by Buddhists in Nepal as an incarnation of the bodhisattva of compassion, Avalokiteshvara. As with most Indian myths, there are many versions of the story of Matsyendra's metamorphosis into a realized adept—and all of them illustrate the radical transformation that yoga makes possible.
"In one popular version, the infant Matsyendra is thrown into the ocean because his birth has occurred under inauspicious planets. Swallowed by a giant fish, he overhears Shiva teaching the mysteries of yoga to his consort Parvati in their secret lair at the bottom of the ocean. Matsyendra is spellbound. After spending 12 years in the fish's belly, all the while exploring yoga's esoteric practices, he emerges as an enlightened master."
She also mentions that Matsyendrasana is one of the few asanas described in the Hatha Yoga Pradipika.
David Gordon White writes cryptically in The alchemical body: Sidha traditions in medieval India:
"In a curious way, Matsyendra is doubly connected, by this name [although he does not seem to explain how], to the lower half of the yogic body, the place of the sleeping kundalini. However, just as the kundalini sleeps at a turning point in the play of divine manifestation and resorption, so too, the symbol system constructed around Matsyendra's name is an ambivalent one. An extensive body of medieval sources connect Matsyndra to his doctrine of the fish belly [again . . . what?] through a mythic gloss of this name. Matsyendranatha means "He Whose Lord is the Lord of Fishes". . ., and his connection with fish is explained through a myth in which the Goddess . . . has asked Siva to teach her the most secret of all esoteric knowledge, things he has never told her before. This Siva agrees to do, but he has barely launched into what will turn out to be the essence of the Hindu tantric teachings than does the Goddess fall asleep. Siva's words do not, however, go unheeded. Matsyendra, who has been swallowed by a fish (whence his name in these sources), draws up to the shoreline and overhears everything."
So what do we take from all of this? For me, the twisting action of Matsyendrasana does provide an opening for kundalini energy to be activated and offers the spine as a vortex of energy or bindu of the mandala which is our own body.
Does anyone else know more? I would love to hear.