Guptasana is given in the second lesson of the Gheranda Samhita, among the thirty two postures that text presents as the useful ones. The feet are concealed between the knees and the thighs and the practitioner sits down upon them. Gupta means hidden, concealed or secret, and the name refers to the feet being out of sight. Nothing about the practice itself is secret, and the atlas states that plainly because the word invites the opposite reading.
The Hatha Yoga Pradipika does not name the posture. It belongs, with muktasana and several others, to the material carried only by the later manual, and within that material it sits in the group of low bound seats rather than among the prone and standing forms that make the Gheranda Samhita's list so much wider than its predecessor's.
Those seats are described with a precision the rest of the corpus rarely matches. Where a heel goes, which foot lies above the other, whether the soles face up or in: these distinctions are what separate one entry from the next, and the compilers evidently thought them worth recording exactly. That care is consistent with the function the manuals assign to a seat, which is to be held still and unbroken through long sequences of breath restraint. A detail that would be trivial in a shape passed through in seconds is not trivial in one held for an hour.