Read David Swenson's reply:
I would not limit your query to apply solely to Ashtanga Yoga but to all forms of yoga. The word yoga means "union," but union of what? The universe in which we reside is comprised of opposing forces. These oppositions permeate our life in all areas of existence, such as day and night, male and female, summer and winter, right and left, birth and death. The principles of yin and yang, meaning ease and effort, is another example of opposing forces. In everyday life we may not think much about this idea, but the yogis dove deeply into the arena of oppositions of force at play in both the internal and external universe.
In the practice of yoga, we seek balance between these forces through the realms of our visible practice and that which we cannot see. In the early stages of yoga practice, we tend to become fixated on the visible world. The physical practice is readily accessible, so that is where we tend to place our focus. This, however, is only the surface of the practice. Over time, our awareness must turn inward to seek the subtle dynamics of practice. This is the world of prana. Our awareness of prana can be cultivated through development of proper breathing and use of bandhas. Every movement must be initiated, guided, and supported with the breath and energy. We could liken this to a bird utilizing the unseen currents of wind beneath its wings in order to glide with little effort.
As we explore this internal world, we will discover that the practice will become less about the use of muscles and physicality and more about the movement of energy. When we feel ourselves working from our musculature and forcing our way through our practice, we can understand that we are creating an imbalance. At this point, slow down and bring awareness to the sound, texture, and quality of the breath. This will be an immediate indicator of the quality of the practice.
The question you have asked is a good one, and one that we should all continue to ask for the lifetime of our practice. There is no point where we can say that we have mastered yoga. We can only seek to identify and intertwine the oppositions of force at play within and without and, in this way, journey toward balance.
David Swenson made his first trip to Mysore in 1977, learning the full Ashtanga system as originally taught by Sri K. Pattabhi Jois. He is one of the world's foremost instructors of Ashtanga Yoga and has produced numerous videos and DVDs. He is the author of the book Ashtanga Yoga: The Practice Manual.