Of the fifteen postures the Hatha Yoga Pradipika describes, fourteen are named for shapes, animals or qualities. One is named for a man. Matsyendrasana carries the name of Matsyendranath, whom the Nath tradition counts among its founding masters and holds to be the teacher of Gorakshanath.
The description has the foot placed at the root of the thigh, the knee taken across the body, and the trunk turned toward it, held with the arm. It is the seated spinal twist, and it is the only rotational posture in the earlier manual. Svatmarama attaches to it the claim that it kindles the digestive fire and puts an end to disease. That is the text's claim and the atlas reports it as such.
The presence of a personal name inside a technical list is a marker of provenance. David Gordon White's study of the medieval siddha traditions in The Alchemical Body sets out the world these texts came from, in which lineages of accomplished practitioners transmitted alchemical, tantric and yogic technique together, and Geoffrey Samuel's account of the emergence of yoga and tantra places the same milieu behind the hatha corpus. A posture bearing a founder's name in the oldest surviving hatha manual is that lineage signing its own work, and it is the strongest internal evidence in the text for where the material came from.