The classical manuals describe limbs. Where a hand goes, where a heel rests, how the legs cross: that is the whole content of nearly every entry. Simhasana is the exception. The Hatha Yoga Pradipika crosses the ankles beneath the perineum with the knees wide, sets the palms on the knees with the fingers spread, and then specifies that the mouth is open and the gaze fixed at the point between the eyebrows.
It is the only posture in the corpus that says what the face is doing, and the only one involving the mouth at all. That singularity is the most interesting thing about the entry, because it shows the compilers were willing to record whatever a posture required and simply had no occasion to describe faces elsewhere.
Simha is the lion. The open mouth and the spread fingers are the animal's roar and claws, and the wide knees complete the crouching shape. Svatmarama counts simhasana among his four principal postures, calls it the seat honoured by the greatest of the yogis, and says that it assists the three locks, the root, abdominal and throat contractions that the corpus treats as central to its breath work. That last statement ties the posture directly into the technical system rather than leaving it as one shape among many.