When we practice cleansing our physical level for spiritual purposes we begin a process of eliminating impurities that accumulated from past unhealthy habits. By fasting and eating a natural diet of whole foods, we enable the body to clean itself. For even deeper cleaning, Yoga practice also includes kriyas, specific techniques that have a purifying effect on the entire system.
In this way, we reawaken our instinctive ability to discriminate between that which is clean and suitable for consumption, and extreme substances that cause imbalance. We redevelop a natural disinclination for things like alcohol, smoking, sugar and caffeine. Physical cleansing lays a foundation for working with the more subtle levels of being, the purification of the heart and mind.