Kriyas in Yoga are purifying processes, which help to free the human body from toxins and, thus, decontaminate the nervous system. In Sanskrit, the word Kriya means "action, deed, effort". These Kriyas are six in number according to traditional Yoga system. The Kriyas include Neti, Dhauti, Basti, Nauli, Shanka-prakshalan, Kapalbhati and Tarataka. For purification process, a student of Yoga can safely practice any of these methods.
Most people usually clean themselves externally to look best, but Yoga believes in external as well as internal cleaning, as the body, mind and soul are reliant on each other. Six fold purposes of Yoga Hygiene or kriyas include removal of diseases, purification of the external and the internal organs of the body; maintenance of highest physical efficiency; through good health; auto-immunization; longevity; mental, moral, and spiritual sublimation. Kriya usually refers to a technique or practice within a yoga discipline and also the outward physical manifestations of awakened kundalini. They are the impulsive movements resulting from the awakening of Kundalini energy. Types of kriya may widely vary between different schools of yoga.
Kriya practices are a part of traditional Hatha Yoga. Certain kriyas eventually developed into the asanas of Hatha yoga and are mentioned as pre-requisites. It is believed that these practices cleanse the body and prepare it for the various postures of Hatha Yoga.
The yogic kriyas or cleansing practices are also called shat-karma (six-actions). The practices of Kriya are quite difficult, sometimes revolting, strange and are not as easy as most yoga postures are. The Yoga gurus usually practice Kriyas and the students seldom perform these during the initial stage. Teaching these practices also requires great knowledge, since there is the risk of harming the student.
The Yogic Kriyas are described in Gheranda Samhita – These are Dhotis, Bastis, Neti, Trataka ,Nauli, Kapalbhati Out of which 21 subgroups are described.