If we look at the entire system of yoga, it is used as a tool, and an end. We constantly lose sight of this goal, throughout the day and during the practices. We have misconceptualized yoga, and need to understand the tools we use are still yoga and the end result is also Yoga. So what is the goal? Training of the mind.
Sometimes the practices look very gross and physical. Take asanas for example, but the true purpose of the practice is to train the mind. Another example are Kriyas (or cleansing practices). Sutra Neti for example, where you stick a piece of thread up your nose and out through your mouth, to cleanse your nasal cavities. Some people feel fear and anxiety just at the thought of doing this. This is due to past conditioning. Society has told us at such a young age to never stick things up our nose. We are re-conditioning the mind with this practice. This challenges the old habit pattern, and begins to re-train the mind.
We are working with our involuntary reflexes and beginning to train them to be voluntary. We are doing this to ourselves, this is an internal action. All the excess mucus and tension in the body begin to release with this practice. The same goes for Vaman. Where you drink 1-2 liters of saltwater on an empty stomach and throw it back out. What happens is all the excess mucus in the stomach comes out. It is actually a rather gentle practice, because you are voluntarily doing this to yourself, versus purging from sickness. Results are very different when we do these practices to ourselves versus having someone else do them to us. For example, internally healing through yoga and its practices versus using external influences such as a psychotherapist. When we do this ourselves, the results are much deeper.
When we begin to re-train our internal system, then external conditions seem to affect us less. Change in seasons for example. When practicing Sutra Neti we can then begin to change the temperature of the water being used. The 6 Kriyas will train our entire nervous system and physiological system. Only performing these internal kriyas will completely cure sinusitis or asthma. When all these are performed, it will leave you with more control, and more control will lead to purification.
There are a few steps in the purification process, 1st on the gross physical body, speech and then thoughts. In each of the practices, we are engaging, forming, and re-training the mind, naturally. Physical and biochemical changes take hold during these practices. Many changes are taking place simultaneously at a multi-dimensional angle. Overall, we are training the mind with each practice, gaining control over our involuntary reflexes then purifying the body, speech and thoughts.