Kundalini is a Sanskrit word that means coil and referes to the non physical energy located in the perineum. Every human has Kundalini, however, in most humans the Kundalini lies dormant in the perineum, forming a 3½ circles coil. Although most people are not aware that the Kundalini exists, Kundalini is well-known by serious spiritual seekers. Most yoga (*) traditions aim to awaken the Kundalini, because these yoga traditions believe that the Kundalini is the power that is required to attain the ultimate goal of spiritual achievement, the union with divine (Yoga).
Traditionally, it is believed that after being awakened, Kundalini will be able to remove the knots of the major chakras, cleanse, open and develop the chakras until they are fully developed into full-blooming lotus forms. By the time the Kundalini finishes opening, cleansing and developing all of the chakras, the core of the Kundalini will reach the crown chakra, the chakra at the top of the head. This is the time you attain the highest spiritual achievement — Yoga.
However, it is not so easy for someone to attain Yoga. First, it is very hard to awaken the Kundalini. Some people have spent decades of dedicated practices without being able to awaken their Kundalini. It is believed that only a few were really able to awaken their Kundalini. For those who were lucky enough to have their Kundalini awakened, the cleansing process of the Kundalini would still have taken decades. Kundalini requires years to cleanse, open and develop a chakra completely, and the process is done in a bottom up in sequential order. So, as the Kundalini starts at the lowest chakra and spends a few years at this base chakra, it will take decades before the Kundalini can start working at the crown chakra, the 7th chakra. In most cases, one will die before achieving the final goal of the process since the time required is too long.
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(*): Yoga tradition here refers to the different spiritual paths that aim
for Yoga (union with divine) as the final goal