The name paschimottanasana is literally the posture of the intense stretch of the west. Paschima means west, and in the anatomical vocabulary of these texts it also means the back of the body. The two senses are joined by an assumption about how a practitioner sits: facing the rising sun, so that east is in front and the spine faces west.
That convention is not incidental. Orientation matters throughout South Asian ritual practice, in the siting of temples, in the direction a householder faces for domestic rites, and in the arrangement of the fire altars of the older Vedic ritual. A technical term for the back of the body built out of a compass direction records a world in which practice was habitually oriented, and it is one of the small places where the hatha corpus shows its inheritance without arguing for it.
The Hatha Yoga Pradipika describes the posture as the legs extended straight, the toes caught in the hands, and the forehead brought toward the knees. The Gheranda Samhita carries it as well, placing it in the stock shared by both manuals. It is one of the few postures in the earlier text that neither sits cross legged nor balances on the arms, which makes it slightly anomalous in a list otherwise composed of seats.