Dhauti and Basti give control over the digestive tract all the way from the mouth to the rectum. Neti and Trataka are responsible for clearing the breathing passages and the vision through steadying the eyes. Kapalabhati and Nauli Kriya are the crowning processes of the six acts. Like any other art or science Hatha Yoga includes both crude and refined methods of cultivation. Article: Shatkriya by Sundernath