Six main kriyas

what are the six main kris and why are they performed ? YTT in training asking (:

Limbs, eyes, ears, nose, mouth etc. can be termed as external organs of the body. There are many more internal organs in our body. We clean our external parts twice or thrice a day or whenever any dust or dirt accumulates on them. We do so because we see them. Then how about internal organs? The dirt accumulated on or in them is not visible, so we don’t pay any attention to keep them clean as we do for our external organs. Cleanliness of internal organs or parts is much more important than the external ones. Our Munis and Yogis have prescribed many processes to clean and remove out the unwanted matter accumulated in the internal organs of the body. These may be done either before or after practicing Yogasanas. There are six major cleaning processes in Yoga. They are:

NETI KRIYAS : Jala (water), Ksheer (milk), Tail (oil), Ghrit (ghee), Sutra (thread), Swamutra (self urine), Gomutra (cow urine), Neti Kriyas (nose cleaning processes).
DHAUTI KRIYAS: Jala (water), Vamana (vomit), Vastra (cloth), Danda (stick), Dhauti Kriyas (stomach cleaning processes).
VASTI KRIYA : Vasti (enema) and Shankh Prakshalan – (large intestine cleaning processes).
NAULI KRIYAS : Agnisar, Uddiyan, Nauli (navel rotation processes for purifying abdominal organs).
KAPAL BHATI : Bhastrika-Kapalbhati (forceful breathing processes for purifying the skull and brain).
TRATAK: Continuous gazing (eye cleaning processes).