There are several mentions of the āsana therm in Gorakhabānī:
आसण बैसिबा पवन निरोधिबा थांन मांन सब धंधा |
गोरषनाथ आतमां विचारंत ज्यूं जल दीसै चंदा || ८२ ||
āsaṇ baisibā pavan nirodhibā thāṃn māṃn sab dhaṃdhā |
badaṃt goraṣnāth ātamāṃ vicāraṃt jyūṃ jal dīsai caṃdā || 82 ||
Seating (baisibā) in posture (āsaṇa), overmaster, calm (nirodhibā) vitality (pavana), external environment (thāṃna), your social image (māṃna), all worldly activities (saba dhaṃdhā).
Gorakshanath says (badaṃta goraṣanātha), “Comprehending oneself or Atman (ātamāṃ vicāraṃta) in the same way that (jyūṃ) the moon (caṃdā) is reflected (dīsai) in water (jala).”
दिढ अहार दिढ जे न्यंद्रा दिढ होई ||
कहै सुणौ रे पूता मरै न बूढा | || ||
āsaṇ diḍh ahār diḍh je nyaṃdrā diḍh hoī ||
goraṣ kahai suṇau re pūtā marai na būḍhā hoī || 125 ||
The position, location (āsaṇa) should be settled, stable (diḍha), the nutrition controlled (ahāra diḍha), whoever (je) sleeps (nyaṃdrā), should be (hoī) regulated (diḍha) in this. Goraksha says (goraṣa kahai), “Listen (suṇau), o (re) my spiritual son (pūtā). Then, will (hoī) not (na) be senility (būḍhā) and death (marai).”
Thanks to Gorakshanātha, much becomes clear with regard to the description of āsana in Yoga Sūtras of Patañjali. I am going to give explanations.
sthira-sukham-āsanam || 46 ||
The position (āsana) must be stable (sthira) [with perfect inner space] (sukha).
The term āsana comes from the root ās, which can have several meanings. The main one is “to sit” or “a seat”, but it can also mean “a place of seating” and “space”. Still it can be translated as “to be”, “to stay” in something or “to live” somewhere, that may indicate the action itself. Thus, it may be a soul staying in the body as puruṣa: ‘pura‘ – a body and ‘uṣa‘ – that is in the body. For example, when we invoke some Deity, we offer him/her a place to sit. So, Nāthas texts often say that the bodies of all living beings were created by Śiva and āsanas too, and āsanas are as many as embodied beings. The main āsanas and number of embodied souls are 84 (siddha-puruṣas or nāthas, who realised in themselves or experienced themselves as Śiva-svarūpa).
Further, the term sukha many people translate very shallow and even in a “sporting way” as “comfortable” or “convenient” posture. However, I think that for a better understanding we should consider the etymology of the term. The term consists of the prefix su सु, which means something “good”, “perfect” and kha ख – “space”. So, if there are blockages in psychophysics, then you are aware of your body as the negative space (duhkha), the opposite of sukha. Practicing āsanas you are freed from blockages and become a pure space, and the purest space is the space of ātman. When your thoughts and body reflect it, they become stable (sthira). All this is what Gorakshanātha spoke about, reflecting on the ātman (it is unchangeable, eternal), the body becomes like water reflecting the moonlight (ātman). If the body reflects the ātman, self nature, then body becomes like it, reflecting its stability.