Sparks of Kabala

The Kavanos while immersing in a Mikveh:[1]

One is to immerse eight times in the Mikveh, each time with a different Kavana. The first time one immerses he is to intend that he is immersing in a Mikveh Taharah which is the Gematria of 151 which corresponds to the name Eh-Yeh. This purifies one from his impure state. The second immersion is to rectify one’s bad trait of anger. The third immersion is to rectify the blemishes he created in Sheim Ab. The fourth immersion is to rectify the blemishes he created in Sheim Sag. The fifth immersion is to rectify the blemishes he created in Sheim Mah. The sixth immersion is to rectify the blemishes he created in Sheim Ban. The seventh immersion is to remove the weekday garments of the soul. The eighth immersion is to receive the ray of Yom Tov, of Yom Hazikaron. These Kavanos likewise apply on Erev Yom Kippur.

The purification affected by a Mikveh:[2]

There is a tradition from the Kabalists that there are four hundred aspects of impurity that reside upon an impure person. In order to remove these 400 aspects of impurity one must immerse in a Mikveh of 40 Seah. The waters represent Chesed and hence have the ability to purify the body. One’s entire body must be immersed in the Mikveh simultaneously as if even one hair is protruding from the water the impurity remains on that hair and then spreads to the rest of the body [upon coming out of the water].