Asana as Mudra
When master yogis, mahasiddhas, go about their daily lives, they naturally manifest the yogic seal (skt. mudra) of the two awakened bodies. These two bodies are; mental body of empty phenomena (skt. dharmakaya) and mental body of playfulness (skt. sambhogakaya). Be it day or night, in peaceful or wrathful circumstances, the minds of mahasiddhas are in perfect harmony and balance. This is the great yogic seal, mahamudra, which entirely concerns nonphysical bodies of man. There is no question about the fact that the foundation of yogic accomplishment is the mind.
In human form, however, seals or mudras can take a further expression of physical postures or asanas. Before delving further into the meaning of asana, the reader should remember that the foundation of all and any type of yogic practice is the nature of mind, in other words, basic wakefulness-kindness-peacefulness. If asana yoga is exercised by practitioners who haven't yet established natural state as their default mode of being and remain in the samsaric state, this needs to be simultaneously adressed during asana yoga by appropriate techniques that enable the practitioner to resume recognition of the two awakened bodies whenever it is lost. Without this, the real meaning of asana as mudra will never be understood as mahasiddhas of the past understood them.
The real meaning of asana is mudra, and the real meaning of mudra is asana. Asanas, regardless of their simplicity or complexity, are physical expressions of the awakened nature of all sentient beings. Also, as is commonly said, asanas are a way to stay healthy and strong. From this perspective we could say that asana yoga is maintenance of our body instrument that we use to live in this world. What is interesting is that contrary to advancement in the yoga of mind*, which is irreversible, one can never achieve a state in physical yoga that would be irreversible. If one doesn't practice asana, be it on yoga mat or in any other form, the condition of the physical body begins to deteriorate.
*purification of mind, advancement in bhumis
Asana as Mudra
A common way that asanas are practiced, is to just do the movements and put one's body into some postures. One tenses, relaxes and stretches the muscles of the body and opens its joints through various applications. From the point of view of keeping the body healthy and strong, this is perfectly sufficient. From the point of view of practicing asana as mudra, however, this is incomplete.
For one to understand what asana as mudra means, one has to meet the following requirements,
have recognition of wakeful nature of one's mind
feel the spontaneous unification of the wakeful mind and the physical body
learn to move the body while recognising the wakeful mind
study the alternation of tensing, stretching and relaxing of muscles while recognising the wakeful mind
realise that there is no difference between tensing, stretching and relaxing of muscles while recognising the wakeful mind
realise that all postures and movements, both in and out the yoga mat, are asana as mudra
I have taught extensively on point 1. how to recognise the wakeful mind so I will simply refer to Pemako-website and youtube, and won't repeat anything here but to introduce this idea, asana as mudra, I will write a bit more on point number two because it is a decisive stage.
Unification of the Wakeful Mind and the Physical Body
All types of yogic physical practices begin from a simple standing posture, known as tadasana or samasthiti. Same is true in Chinese yogic traditions.
The real meaning of tadasana is to have wakeful mind and physical body unified, to study the connection and relation of the two, and to learn to adjust the muscles and joints in a subtle manner that enables staying in this posture for long durations (up to 30 minutes), if so wanted, without any discomfort.
To those who want to practice asanas purely for physical benefits, standing in stationery posture might sound pointless. However, in this stage one makes a wonderful discovery after the other about how the body becomes unified, in the midst of basic wakefulness. One's whole being becomes full of delightful light and subtlest of blisses (skt. sahajananda) often discussed in the writings of mahamudra masters. This stage of learning is demanding because it takes some time and effort to strengthen the internal muscles of the body and consequentially feel unification that can be described as sense of unified relaxed strength, that feels as if the body was weightless.
All right, I think this is enough for now. Feel free to contact me if you have questions. I'm happy to share my understanding of asanas.