In the Hatha Yoga Pradipika the cockerel stands immediately before uttana kurmasana, the stretched or upturned tortoise, and the tortoise is described as being entered from it. Holding the lifted lotus, the practitioner takes hold of the neck and rolls back onto the spine, so that the bound legs end up above a body lying on its back.
That is one of very few places in the classical corpus where a text presents one posture as the doorway into another. The manuals otherwise list their postures as independent items, with no ordering, no sequence and no instruction about moving between them. The absence is significant for anyone comparing the medieval literature with modern practice, where linked sequences are often the organising principle of a whole method. Mark Singleton's Yoga Body traces the formation of those sequences in the twentieth century, and the classical texts give them very little to build on.
The pairing also shows the compilers thinking in terms of the body's mechanics rather than of symbolism. A cockerel and a tortoise have nothing to do with one another as animals. They are joined here because one arrangement of the limbs converts into the other by a single movement, and the text records that fact. Read together, the two entries are a small piece of evidence that the posture lists were assembled by people who had done the postures, not only copied them.