Kundalini is the serpent power or sleeping Sakti, that has 3 1/2 coils with face downwards, in the Muladhara Chakra, at the base of the spine. No Samadhi is possible without its being awakened. The practice of Kumbhaka in Pranayama produces heat and thereby Kundalini is awakened and passes upwards along the Sushumna Nadi. The Yogic practitioner experiences various visions. Then the Kundalini passes along the Six Chakras and eventually gets united with Lord Siva, seated on the Sahasrara or thousand-petalled lotus, at the crown of the head. Nirvikalpa Samadhi ensues now and the Yogi gets liberation and all the divine Aishvaryas.
One should practise control of breath with concentration of mind. The awakened Kundalini that is taken up to Manipura Chakra may drop down again to Muladhara. It has to be raised again with effort. One should become perfectly desireless and should be full of Vairagya before he attempts to awaken Kundalini. Kundalini is like a thread and is resplendent. When it is awakened it hisses like a serpent beaten with a stick and enters the hole of Sushumna. When it travels from Chakra to Chakra, layer after layer of the mind becomes open and the Yogi acquires various Siddhis (psychic powers).
Chakras are centres of spiritual energy. They are located in the astral body, but they have corresponding centres in the physical body also. They can hardly be seen by the naked eyes. Only a clairvoyant can see with his astral eyes. Tentatively they correspond to certain plexuses in the physical body. There are six important Chakras. They are: Muladhara (containing 4 petals) at the anus; Svadhishthana (6 petals) at the genital organ; Manipura (10 petals) at navel; Anahata (12 petals) at the heart; Visudha (16 petals) at the throat and Ajna (2 petals) at the space between the two eyebrows. The seventh Chakra is known as Sahasrara, which contains a thousand petals. It is located at the top of the head. Sacral plexus tentatively corresponds to Muladhara Chakra; Prostatic plexus to Svadhishthana, Solar plexus to Manipura, Cardiac plexus to Anahata Chakra, Laryngal plexus to Visuddha Chakra and Cavernous plexus to Ajna Chakra.
The Science of Pranyama by Sri Swami Sivananda