What is kundalini?

In Kundalini Yoga, it is said there is a snake coiled at the base of the spine. But what is this snake in terms of subtle body anatomy? At first glance it doesn’t make sense. What the hell is this extra phenomenon? A snake at the base of the spine?
I prefer not to think of Kundalini as an energy separate from prana. Instead I see Kundalini as a description of the alchemical process of purifying and opening the spinal channel, allowing more and more prana to rise and circulate. And when this is done gradually, there is no hot snake burning up the spine. The experience is more like what is described in Taoist alchemy. It can even feel like a pleasant, cool energy rising up the spine, carefully releasing blockages over time. This view makes it much easier to understand how Taoist and Hindu systems are describing similar experiences.

From The Heart of Yoga by T. K. V. Desikachar
“The concept of kundalini is confused by many imprecise definitions, and even a text such as the Hatha Yoga Pradipika contains contradictory descriptions of it. The definition that follows is derived from what in my opinion is the best, the clearest, and the most coherent text on this subject, the Yoga Yajnavalkya. There kundalini is defined unambiguously as an obstacle. What is to enter the sushumna at some stage or other through your yoga practice is, according to this text, not the kundalini itself, but simply prana. Many books say that it is the kundalini itself that rises up through the sushumna, but this does not make sense if we follow the Yoga Yajnavalkya, one of the oldest texts that deals with this aspect of yoga. One of its central concepts is that prana and the various forms it takes in the body are linked to the practice of yoga, and it says that if we are successful in our practice, the kundalini is burned up, making the way clear for prana.

A snake killed while lying in a curled position unfolds and stretches out, the muscles no longer able to keep it coiled. It is said that when the fire in the body, agni, has killed the snake, the kundalini unrolls and the passage is open to the flow of prana. This does not happen overnight. Even when parts of the kundalini are destroyed, it remains capable of blocking sushumna for a long time.

If you closely consider this image, it becomes clear that kundalini is another way of depicting what we call avidya. In the same way that avidya can become so powerful that it totally prevents us from seeing purusa, kundalini blocks the prana and prevents it from rising through the sushumna. The moment the kundalini is burned is the same moment that avidya ceases to exist. Then prana is able to enter the sushumna and slowly move upward. We can also understand hatha yoga as part of raja yoga, which is defined as the process in which prana, the friend of purusa, gradually rises upward. When it gets to the top, purusa unfolds and the king within us emerges. When the emphasis is primarily on the concept of kundalini, then we speak of the practice as kundalini yoga. Hatha yoga is so named when our practice focuses on removing the division between ha and tha.”