Matsyendranath is a figure of tradition rather than of documented biography. The Naths hold him to be the first human teacher of their line, the recipient of teaching from Shiva himself, and the master of Gorakshanath, who is credited with organising hatha yoga into a system. His name means lord of fishes, and the stories told about him include the account of his overhearing Shiva's instruction to Parvati while in the body of a fish. Dates, place and historical detail are not recoverable from the sources, and the atlas does not supply them.
What is recoverable is the shape of the tradition around him. David Gordon White's The Alchemical Body reconstructs the medieval siddha world in which the Naths operated, where mercurial alchemy, tantric ritual and yogic physiology were pursued as one enterprise aimed at a perfected and immortal body. Geoffrey Samuel's Origins of Yoga and Tantra situates the same movement in the longer development of Indic religion.
The hatha corpus is the technical residue of that world. It reaches later readers as a set of manuals about postures, breath and inner locks, with the metaphysics largely stripped out, and the personal names embedded in it are among the few traces left on the surface of the texts of where the material came from. Two postures in the classical corpus carry Nath names, this one and gorakshasana, and they carry the teacher and the disciple respectively.