The Ashtanga practice which was systemised by Sri K Pattabhi Jois, is made up of six set sequences which are designed to purify the body, nadi or nervous system, the sense organs and the mind so the seeker may find his true self. There are a total of six sequences in the Ashtanga system, Primary, Intermediate, Advanced A, B, C, and D. Each sequence is learnt one asana at a time.
Primary series is called Yoga Chikitsā, meaning Yoga therapy and it is designed to heal and purify the body of the practitioner.
Intermediate series is called Nadi Shodana, meaning Nadi or nerve cleansing and is designed to purify the subtle energy channels knows as nadis, so by prana can flow freely throughout the body.
Advanced A, B, C and D is known as Sthira Bhaga meaning strength and grace. Advanced deepens everything that has been learnt in both the Primary and Intermediate series.
Pattabhi Jois emphasised that Viñasa and Tristasana as the main components of Ashtanga Yoga.
Viñasa is a breathing and movement system. For each movement there is one breath, so movement and breath become synchronised. Each asana has a set number of viñasas which are learnt. For example we can look at Suryanamaskara. The first viñasa is "Ekam" Inhale take the arms above the head. The second viñasa is "Dwe" exhale fold forward and bring the hands to the floor, and so on.
This breathing and movement system known as viñasa causes internal cleansing to take place. The controlled vinasas in asanas causes internal heat and as Pattabhi Jois would say, it would make the blood "boil". This heat created from the practice thins the blood so it circulates around the joints removing any pain as well as moving through the organs and removing impurities, which are brought out of the body through sweat.
Tristasana as the name suggests, has three elements. These are the three areas of focus within the Ashtanga Practice.
1. ASANA (the posture)
3. DRISHTI (focus point, where to look)
These three components should always be performed together. Together they purify the body nervous system and mind.