One of the most immediate and obvious aspects of Ashtanga yoga is the emphasis on the physical postures, the asanas. It is one of the more quick paced styles of yoga, and is often described as 'power yoga'. But that is just when on the outside looking in on the practice, and to call it that robs the practice of it's essence. Once immersed in the practice it is the invisible content that is of importance. This content consists of the Ujayi breath (a specific breath technique), Bandhas (internal energetic locks within the body) and driste (gaze). This tristana forms the basis of Ashtanga yoga.
The breath is one of the main foundations of this practice and links each posture together in a precise order. The importance of the breath cannot be over-emphaisised. One of my first yoga teachers told me that it was the quality of the breath that distinguished a newcomer to yoga from a more experienced yogi. Not how flexible the body was. But the quality of the breath. And in ashtanga we synchronize our breath with our movements, and engage Mula and Uddiyana Bandhas to produce an intense internal heat. This heat cleanses the body, making it more flexible, stronger and healthier.
There are three series in the Ashtanga system. The Primary Series (Yoga Chikitsa) detoxifies and aligns the body. The Intermediate Series (Nadi Shodhana) purifies the nervous system by opening and clearing the energy channels. The Advanced Series A, B, C, and D (Sthira Bhaga) integrate the strength and grace of the practice. Each series is to be completed before proceeding to the next, in the same way that the sequential order of asanas within each series is to be followed. Each posture is a preparation for the next, developing the strength and flexibility that is required in order to move on. The practice is designed to gradually open up blockages inside the body/mind/nervous system and awaken the practitioner to a greater sense of space and freedom within. So it's not about the physical practice, but the asanas in themselves are a step in starting to transcend the body/mind/nervous system.
Traditionally in ashtanga we practice 6 days a week, and for most practitioners each practice will be the same postures, in the same sequence. This sometimes gives rise to the question of getting 'bored' with doing the same practice every day. But every day my practice is different, as my body is different every day. No two practices are ever the same for me. And my body gets to know the postures intimately, allowing my mind to stop charging ahead, and on a good day, my practice becomes a moving meditation.
But as Sri K. Pattabhi Jois likes to say "99% Practice and 1% Theory". In other words "Practice and all is coming"