Neck as Sushumna Central Spinal Passage (Nadi)
By convention, the first accomplishment in kundalini yoga is to bore out and keep open the sushumna nadi central spinal passage. This has to happen in the subtle body but it is practiced by actions in the physical form. The most common and least reliable method for this is visualization. That is for those who have not stumbled upon a reliable method, which is the fourth step of ashtanga yoga, pranayama breath infusion. Step number one is to infuse the breath into the lungs so that it accumulates and can be pushed down the front of the body so that it mixes with the reproduction hormone force which is in the front part of the lower torso. Once this energy mixes it has to be compressed inwards upon itself and forced down and under to the back so that it will strike muladhara base chakra where kundalini reposes itself.
If the student is successful kundalini will be aroused and will try to move in one direction or the other but it will be restricted as to where it could travel by the lower locks. These are the anus, sex and navel locks. With these locks applied, kundalini will be forced upwards through the spine, and will behave properly if the neck lock is applied while it passes through the spinal column in the neck. The result, if kundalini has enough impetus, is that it will pass into the head of the subtle body where it will spread itself in any direction it so pleases unless the student already applied the mind lock which essentially is central focusing of the self on itself or focus between the eyebrows with a firm neck lock applied.
If the student feels as it he or she will lose it, meaning lose control of the physical body, then the student should applied the locks even firmer while gradually bringing the body down the ground but without releasing any locks and not in any way causing the spine to collapse or lose posture. The eyes should be closed so that the student can focus fully inside the psyche.
By repeatedly doing this, day after day for some time, for years if necessary, the student will graduate so that he or she can begin the practice of cell kundalini. For this type of practice, the objective will no longer be to force kundalini through the spine as the only objective. Cell kundalini is concerned with burst of kundalini energy where it is without it going here or there. It could be also burst of many cells in their particular region combined where they impel on themselves or implode on themselves. This causes the sushumna nadi central spinal passage to lose significance. Then the entire neck acts as the sushumna nadi central passage. See these diagrams:
