(As described by Swami Sivananda)
Kundalini Yoga focuses on methods for arousing the dormant kundalini shakti, and techniques for achieving union of Shakti with Siva in the topmost Sahasrara chakra. In this practice, a variety of techiniques are used to move energy up through several different asanas, prananyamas, mantras and mudras.
THE NATURE OF KUNDALINI
Shakti, divine Energy, has two aspects, the static and the dynamic. One cannot have Shakti in the dynamic form without having it in a static form, much like the poles of a magnet. Such a polarity is necessary for maintaining equilibrium in the universe. The cosmic Shakti manifests in the human body as kundalini and prana. Kundalini is the static support for the moving vital forces.
The human being is the microcosm, or inferior cosmos. Whatever exists in the outer universe exists in the body. Shiva resides in the Sahasrara chakra at the crown of the head. Kundalini resides in the Muladhara chakra at the base of the spinal column. Muladhara means 'root-support’. Kundalini is the primordial energy that lies in the basal Muladhara chakra in a dormant, potential state. It is the cosmic power in individual bodies. It is not a spiritual force, but a spiritual potential. It is an electric fiery, occult power, the mighty pristine force which underlies all organic and inorganic matter. It is called the Serpent Power, because of its spiral-like working in the body of the yogi. When awakened, it makes the hissing sound of a serpent and proceeds to rise upwards through the central canal within the sushumna. The rousing of the Kundalini Shakti and Her union with Lord Siva in the Sahasrara brings about a state of supreme consciousness and spiritual experience.
THE ASTRAL TUBES AND THE CENTRES OF SPIRITUAL ENERGY
Every portion of the physical body is associated with an astral counterpart. They are intimately connected and interdependent. All six centres, or chakras, and the Sushumna nadi are in the astral body. In the physical body, these chakras and sushumna nadi coincide with the nerve plexuses and the spinal column.
There are 72,000 nadis or astral tubes, in the pranic sheath. To advance in kundalini yoga, the purification of these nadis is essential.
There are two nadis on either side of the Sushumna known as Ida and Pingala; they correspond to the left and right sympathetic nervous cords in the physical body. When the prana flows through Ida and Pingala, the person is busily engaged in manifold worldly activities, bound by time, space and causation. Sushumna nadi, the central one, is the most important. When the prana is in the Sushumna, the yogi is beyond all limitations of mind and time.
Pingala is associated with solar energy. The word pingala meaning "tawny" in Sanskrit. Pingala has a sunlike nature and male energy. Its temperature is heating and courses from the left testicle to the right nostril.
Ida is associated with lunar energy. The word ida means "comfort" in Sanskrit. Ida has a moonlike nature and female energy with a cooling effect. It courses from the right testicle to the left nostril.
The Ida and Pingala nadis are often seen as referring to the two hemispheres of the brain. Ida nadi controls all the mental processes while pingala nadi controls all the vital processes. Ida is the introverted, lunar nadi, and refers to the right hand side of the brain. Pingala is the extroverted, solar nadi, and corresponds to the left hand side of the brain.
The Sushumna extends from the muladhara chakra, located in the second vertebra of the coccyx region, to the brahmarandhra; the opening of the skull. Western anatomy recognizes a central canal in the spinal cord made up of grey and white brain matter. The spinal cord is suspended in the hollow of the spinal column. In the same way, the sushumna, within the spinal column, has subtle sections. Its colour is red like fire. The sushumna pierces all lotuses and is pure intelligence.
Within the sushumna there is a nadi by the name of vajra which is as lustrous as the sun with rajasic qualities. Again, within this vajra nadi, there is another nadi, the chitra, of sattvic nature and pale in colour. The qualities of fire (agni), the sun (surya), and the moon (chandra) are the three aspects of Brahman. Within this chitra, there is a very fine, minute canal though which the kundalini, when awakened, passes from the muladhara to the sahasrara chakra. In this canal exist all the main chakras, each representing a different state of consciousness.
The lower extremity of the chitra nadi is the door of Brahman. The kundalini passes through this door to its termination point in the cerebellum. The chitra is the highest and most beloved of all nadis. Brilliant with five colours, it is the centre of the sushumna, and is called the Heavenly Way. Another description says that "the chitra is beautiful like a chain of lightning and shines like a lotus in the mind of the sages. She is extremely subtle, the awakened pure knowledge, the embodiment of all bliss, whose nature is pure consciousness".