The "Ojas" is that which makes the difference between man and man. The man who has much Ojas is the leader of men. It gives a tremendous power of attraction. Ojas is manufactured from the nerve-currents. It has this peculiarity: it is most easily made from that force which manifests itself in the sexual powers. If the powers of the sexual centres are not frittered away and their energies wasted (action is only thought in a grosser state), they can be manufactured into Ojas. The two great nerve-currents of the body start from the brain, go down on each side of the spinal cord, but they cross in the shape of the figure 8 at the back of the head. Thus the left side of the body is governed by the right side of the head. At the lowest point of the circuit is the sexual centre, the Sacral Plexus. The energy conveyed by these two currents of nerves comes down, and a large amount is continually being stored in the Sacral Plexus. The last bone in the spine is over the Sacral Plexus and is described in symbolic language as a triangle; and as the energy is stored up beside it, this energy is symbolised by a serpent. Consciousness and subconsciousness work through these two nerve-currents. But superconsciousness takes off the nerve-current when it reaches the lower end of the circuit, and instead of allowing it to go up and complete the circuit, stops and forces it up the spinal cord as Ojas from the Sacral Plexus. The spinal cord is naturally closed, but it can be opened to form a passage for this Ojas. As the current travels from one centre of the spinal cord to another, you can travel from one plane of existence to another. This is why the human being is greater than others, because all planes, all experiences, are possible to the spirit in the human body. We do not need another; for man can, if he likes, finish in his body his probation and can after that become pure spirit. When the Ojas has gone from centre to centre and reaches the Pineal Gland (a part of the brain to which science can assign no function), man then becomes neither mind nor body, he is free from all bondage.
The great danger of psychic powers is that man stumbles, as it were, into them, and knows not how to use them rightly. He is without training and without knowledge of what has happened to him. The danger is that in using these psychic powers, the sexual feelings are abnormally roused as these powers are in fact manufactured out of the sexual centre. The best and safest way is to avoid psychic manifestations, for they play the most horrible pranks on their ignorant and untrained owners.
To go back to symbols. Because this movement of the Ojas up the spinal cord feels like a spiral one, it is called the "snake". The snake, therefore, or the serpent, rests on the bone or triangle. When it is roused, it travels up the spinal cord; and as it goes from centre to centre, a new natural world is opened inside us — the Kundalini is roused.
Quoted from Swami Vivekananda, Lessons on Raja Yoga (1913)