Sushumna nadi is regarded as a hollow tube in which there are three more concentric tubes, each being progressively more subtle than the previous one. The tubes or nadis are as follows: sushumna - signifying tamas, vajrini - signifying rajas, chitrini - signifying sattva and brahma - signifying consciousness. The higher consciousness created by kundalini passes through brahma nadi.
When kundalini shakti awakens it passes through sushumna nadi. The moment awakening takes place in mooladhara chakra, the energy makes headway through sushumna up to ajna chakra.
Mooladhara chakra is just like a powerful generator. In order to start this generator, you need some sort of pranic energy. This pranic energy is generated through pranayama. When you practise pranayama you generate energy and this energy is forced down by a positive pressure which starts the generator in mooladhara. Then this generated energy is pushed upward by a negative pressure and forced up to ajna chakra.
Therefore, awakening of sushumna is just as important as awakening of kundalini. Supposing you have started your generator but you have not plugged the cable, the generator will keep running but distribution will not take place. You have to connect the plug into the generator so the generated energy can pass through the cable to the different areas of your house.
When only ida and pingala are active and not sushumna, it's like having the positive and negative lines in your electrical cable, but no earth. When the mind receives the three currents of energy all the lights start working, but if you remove the earth wire, the lights will go down. Energy flows through ida and pingala all the time, but its effulgence is very low. When there is current flowing in ida, pingala and sushumna, then enlightenment takes place. This is how you have to understand the awakening of kundalini, awakening of sushumna and the union of the three in ajna chakra.
The whole science of kundalini yoga concerns the awakening of sushumna, for once sushumna comes to life, a means of communication between the higher and lower dimensions of consciousness is established and the awakening of kundalini occurs. Shakti travels up sushumna to become one with Shiva in sahasrara.
Kundalini awakening is definitely not fictional or symbolic; it is electrophysiological! Many scientists are working on this, and Dr. Hiroshi Motoyama of Japan has developed a unit by which the waves and currents of energy which accompany the awakening of kundalini can be recorded and measured.
When the roots of a plant are watered properly, the plant grows and its flowers bloom forth beautifully. Similarly, when kundalini awakening occurs in sushumna, awakening occurs in all the stages of life. But if awakening only occurs in ida or pingala or in one of the other centers, it is by no means complete. Only when kundalini shakti awakens and travels up the sushumna passage to sahasrara is the entire store of higher energy in man unleashed. Kundalini tantra.