Makarasana is described in the second lesson of the Gheranda Samhita: the body lies prone with the chest against the ground, the legs stretched out behind, and the head held in the crossed arms. The text claims that it increases the heat of the body. That is the text's statement and the atlas records it as such.
The animal is less straightforward than the usual English rendering suggests. Makara is regularly translated as crocodile, and a crocodile is close enough to what a prone body with its head raised on its forelegs resembles. But in Indian art and iconography the makara is a composite water creature, part crocodile and part fish and often given an elephant's trunk, that serves as the mount of the river goddess Ganga and of the sea god Varuna. Carved makaras flank temple thresholds and arch over doorways in the ornament known as the makara torana, so the creature is one of the most frequently represented figures in the built religious landscape of South Asia.
The Hatha Yoga Pradipika does not carry the posture. It belongs with the crocodile, the locust, the camel and the serpent to the group of prone forms that the later manual adds, and those four together are the strongest sign that the Gheranda Samhita is working from a wider range of bodily arrangements than its predecessor recorded.