In addition to the seven chakras of the subtle body, the Tantras have described a network of subtle channels known as nadis through which the life force (prana) circulate. Nadi means "stream". According to the tantric treatise Shiva Samhita, there are fourteen principal nadis. Of these, Ida, Pingala and Sushumna are considered the most important.
Ida is the left channel. Ida is white, feminine, cold, represents the moon and is associated with the river Ganga (Ganges). Originating in Muladhara, Ida ends up in the left nostril.
Pingala is the right channel. Pingala is red, masculine, hot, represents the sun and is associated with the river Yamuna. Originating in Muladhara, Pingala ends up in the right nostril.
Sushumna is the central channel and is associated with the river Saraswati. Within the Sushumna nadi there are three more subtle channels: Vajra, Chitrini and Brahma nadi through which Kundalini moves upwards running up the body from just below Muladhara chakra to Sahasrara chakra at the crown of the head.
The kanda in Muladhara chakra is the meeting place of the three main nadis and is known as Yukta Triveni (Yukta: "combined", tri: "three", veni: "streams"). In Muladhara, Shakti, the static unmanifested Kundalini, is symbolized by a serpent coiled into three and a half circles around the central axis Svayambhu-linga at the base of the spine. The serpent lies blocking the entrance to Sushumna, the central channel with his mouth. Sushumna remains closed at its lower end as long as Kundalini is not awakened.
The technique of Kundalini Yoga consists in using Prana (the vital air), guiding its circulatory movement through Ida and Pingala down to the base of the spine into the space where Kundalini lies coiled. The vital energies of the opposite forces circulating in Ida and Pingala will be unified and Shakti Kundalini will then awaken and rise up Sushumna, energizing the seven chakras.
From Muladhara chakra, Ida and Pingala alternate from the right to left sides at each chakra until they reach Ajna chakra where they meet again with Sushumna.
In Ajna chakra the meeting of the three main nadis is called Mukta Triveni (Mukta: "liberated"). Continuing beyond Ajna chakra, Ida and Pingala end in the left and right nostrils respectively.
Once the Kundalini Shakti has ascended through Sushumna to Sahasrara, the highest psychic center at the crown of the head, it is made to reverse its course and return to rest in the base center again.