Yoga scriptures say that the prana-shakti or life force called kundalini dwells at the base of the sushumna, the spinal column, in the muladhara or base chakra of each individual from where it controls the entire physiological system through its network of 72,000 nerves.
There are two dimensions to the prana-shakti: at the external level, the life force operates through the sense organs and takes care of our worldly lives; at the internal or spiritual level, usually the kundalini remains dormant.
Kriya yoga consists of a number of pranayam-based techniques that help in hastening the process of spiritual evolution through awakening of the dormant kundalini with controlled breathing. The practitioner of kriya yoga endeavours to make the pranic energy revolve upward and downward, around the six spinal centres or chakras situated in the sushumna muladhara, swadhishthana, manipura, anahata, vishuddha and ajna.
Within the human body, or what is referred to as the pinda, is contained the entire cosmos or Brahmanda: the first five chakras, from muladhar to vishuddha, are the centres of pancha-bhutas or the five elements earth, water, fire, air and ether. The sixth chakra, ajna, is the seat of the mind, while the seventh, the sahasrara, is located in the brain, and represents Shiva, the supra-causal state of Consciousness. The seven lokas corresponding to the seven chakras are bhu, bhuvah, swarga, maha, jana, tapa and satya.
Paramhansa Yogananda says "One-half minute of revolution of energy around the sensitive spinal cord of man effects subtle progress in his evolution; that half-minute of kriya equals one year of natural spiritual unfoldment... One thousand kriyas practised in 8.5 hours give the yogi, in one day, the equivalent of 1,000 years of natural evolution or 365,000 years of evolution in one year."
Yogananda was ordained by the will of the great Mahavtar Babaji the ever-youthful yogi of the Himalayas to share the ancient kriya yoga technique of attaining Self-realisation with the world.
Through kriya yoga the awakened kundalini renders the pinda pure by piercing the chakras. The seeker is then able to enter into deep meditation spontaneously and begins to experience mystic phenomena associated with higher levels of Consciousness.
A stage comes in meditation when the seeker starts hearing rapturous divine music called anahada nada that manifests in 10 different forms: chin-chin, chinchina, the bell, the conch, the veena, the cymbals, the flute, the mridang, the kettledrum, and finally, thunder or meghanada.
The meghanada, accompanied with the sound of AUM, leads the seeker to nirvikalpa samadhi, the supra-causal state of Consciousness, where the awakened kundalini finally unites with Shiva in sahasrara. The seeker then begins to perceive the scintillating neel bindu, the seed of the universe, which is surrounded by a golden halo tinged with saffron.
This sublime experience bestows upon the seeker spiritual enlightenment and realisation dawns upon him that both the perceived, the external universe, and the perceiver, the individual Self, are manifestations of the same Consciousness and that just as innumerable bubbles and waves of the ocean are inseparable from the ocean, so also the entire universe with myriad names and forms emanates from pure Consciousness and is no different from it.