What is asana siddhi and importance?
It would be better to back it up with references from the vedic literatures.
Also it would help to provide some scriptures that mention the importance and also may be the method to achieve.
Asana Siddhi in other words Mastery or achieving perfection in initial posture or sitting position before starting of meditation. Now there are many Asanas like lotus posture , Sukha posture in yoga system. But this term Asana Siddhi is not particularly associated with those types asanas we know , but related to basic sitting posture before starting of mediation or in other words mastering meditation posture.
Asana is the third rung in the ladder of the practice of yoga. If the yamas and niyamas are the foundation of yoga, asana may be regarded as its threshold. 'Asana', literally, means a seat. Here 'seat' does not mean a cushion or some such thing that is spread on the ground. Asana is a pose of the body or the posture which it assumes at the commencement of the practice.
Asana is the initial step in yoga, whereby the bodily structure is set in unison with the cosmos.
The asana should be effortless. There should be no effort not only in the body but also in the mind. Absolute ease of relaxation is the sign of perfected asana. The student should be in a most natural condition in which he is not conscious even of his breathing.
If there is pain, jerk, or a pinching sensation, it should mean that the asana is not properly fixed. There is a prescription given by Patanjali to quicken fixity of posture. And that is 'attention on the infinite'. When this bodily control is achieved, there comes freedom from the onslaught of what are called the 'pairs of opposites', such as heat and cold, hunger and thirst, joy and grief, and so on. Anything that creates a tension in one's system is a pair of opposites. These are overcome by a perfected practice of asana.
Maharshi Patanjali in his Yoga Sutras explaning asana as a comfortable , steady, ralaxed posture. He is not specifying any particular poses or Asanas like Padmasana or Sukhasana etc., but stressing the point that how we should master the seat for meditaion.And this mastering or perfedting an art of sitting properly for meditaion is called as "Asana Siddhi" . Asana Siddhi is essential for achieving concentration , dicipline and making our mind ready for meditation.
Posture (asana) is to be seated in a position which is firm but relaxed.
And how to achieve this perfection of the seat ? Patanjali is also giving answer to this question
Posture becomes firm and relaxed through control of the natural tendencies of the body, and through meditation on the infinite.
A good natural posture is very rare. Most people hold themselves badly and are subject to all sorts of physical tensions. Asana must therefore be perfected by careful training. The aim is to achieve an effortless alertness, in which the body is perfectly steady and yet perfectly relaxed. Since a maladjusted body only expresses a tense and restless state of mind, we are told to calm our minds by meditating on what is infinite. Our minds are incapable of imagining the infinite Brahman; but instead, we can think of the limitless expanse of the sky.
As you have enquired the Importance of Asana Siddhi is also mentioned in Yoga Sutras No. 48
ततो द्वङ्द्वानभिघातः ॥४८॥
tato dvaṅdva-an-abhighātaḥ ||48||
Thereafter, one is no longer troubled by the dualities of sense-experience.
That is to say, by what the Gita calls" the pairs of opposites," the apparent dualities of the phenomenal world—such as heat and cold, pleasure and pain, good and evil, etc. Such complete mastery of the body does not, of course, come through posture alone. It arises from a state of absorption in the consciousness of God. Patañjali goes on to describe the further practices which are necessary in order to reach this state. .
So in conclusion Asana Siddhi is perfecting or mastering basic initial sitting posture for doing practice of meditation. perfecting this initial stage is very important in Yogic Sadhana because it prepares us for the meditation by removing all the dualities from our mind , making our mind stable by removing stress and tension which makes an easy way for concentration of the mind which is necessary for mediation .