There are several essential elements to the practice of asana noted in the Yoga Sutras. What is surprising is that there are only 3 of 196 sutras dedicated to asana.

Asana is perfect firmness of body, steadiness of intelligence and benevolence of spirit.
sthira sukham asanam 2.46
firm, relaxed, steady, steadfast, lasting
sukham happiness, delight
asanam posture

So when we practice, we are working towards, we are practicing being, firm, steady, stable both physically and mentally. We are also practicing being kind. The more we practice, the more we are able to access a stability, a clarity, an open channel of goodwill, kindness, and generosity. The more we practice finding stillness with in the poses, the clearer this channel of benevolence flows.

Perfection in an asana is achieved when the effort to perform it becomes effortless and the infinite being within is reached.
beingprayatna shaithilya ananta samapattibhyam 2.47
 perservering effort, continued exertion, endeavour
saithilya relaxation
ananta endless, boundless, eternal, infinite
samapattibhyam contempletion, conclusion, returning to Source

We practice to connect to something, call it the Infinite, The Field, The Source, God Almighty, that is bigger then ourselves, beyond our capacity to truly grasp. In recognizing that we are part of this energy, we can then begin to surrender into it, relax and be held by it. We can then begin to recognize the connection that we share and let go of control and truly begin to trust.

From then on, the practitioner (the sadhaka – one you does sadhana or practice) is undisturbed by dualities
tatah dvandva anabhighata 2.48

tatah from that, then
dvandvah dualities, opposities
anabhighatah cessation of disturbance

As we practice and find “perfection” or mastery in a pose, we then begin to find a space between opposites thats broad and vast. As Miles Davis says, “I look for the notes not to play”, as yogis we are looking for spaces between the sensations, the feelings, the thoughts, the actions. Once we “achieve” that – perhaps its a split second, perhaps its several minutes of uninterrupted bliss, we can rest, abide in calm, be absorbed, find peace. And it is here, were we can feel or find a sense of freedom that flows unhindered, a peace that is undisturbed by the fluctuations and changes we encounter in our daily lives. We experience a non-dual way of being.

Asana postures are meant to be still and comfortable. Mastery or perfection of a pose occurs when there is effortlessness. From this effortless we experience a state of flow. We don’t simply float on a sea of bliss, we are fully engaged, yet fully relaxed.

Sutras and Sutra definitions are from Light on the Yoga Sutras, BKS Iyengar