The Path of the Kundalini
The progress of one’s spirituality can be tracked along the sushumna. The sushumna is the most important channel in the subtle body. This runs from, what in the physical body would be, the base of the spine to the top of the head. At the bottom of the sushumna is the kundalini (coiled-up), the bio-energy. This is the coiled-up presence of Shakti in the individual being. It is Shakti that throws the individual souls down from Brahman, and it is Shakti that will pull those souls back up to Brahman when she is awakened once more. In most people, the kundalini is never awakened.
Spiritual practices are designed to awaken the kundalini through feelings of love, devotion and will, and thus put one into samadhi. Savikalpa samadhi is achieved when the kundalini reaches the fourth of the seven chakras, or energy transformers connecting the subtle and physical bodies, on the sushumna. In savikalpa samadhi, there is still a little vibration in the mind, so some duality still remains. The deeper the samadhi, the less significant is the vibration. The deepest samadhi, the non-dual nirvikalpa samadhi, the realisation of Brahman, is achieved when the kundalini reaches the seventh chakra at the top of the sushumna. At this point, there is no more duality as the mind has been completely dissolved. There is only Brahman, One without a second. So rare is this samadhi that it is normally said that, in a cycle of twelve years, only one or two people normally achieve it.