In the Yoga Sutras of Patanjali, sutra 2.46 reads: sthira sukham asanam
STHIRA: steadiness, firmness, settling, grounding
SUKHAM: ease, lightness, contentment
ASANAM: seat, posture
Postures should have the quality of steadiness + ease.
Simply that. Take your seat. Be settled but easy.
We make a great big deal out of putting our leg behind our head or standing on our hands. But YOGA practice is ultimately training for sitting. ASANAS (postures) are for cultivating an equal balance of settling and softening. The more often we practice that when we're in challenging shapes, the easier it will be when we take the SEAT.
Asana is the gateway to MEDITATION. Asana is not the means to flexibility or joy or a complete self. It is designed to improve your ability to SIT. Happy practice!