Most of you are familiar with the symbol or diagram of ajna chakra, which has two semicircular petals attached to another circle in the middle. On the left side is written the mantra Ham, on the right Ksham, and in the centre Om. These mantras represent the shabda or vibration of ida, pingala and sushumna nadis.
In scientific terminology, you may relate these to the parasympathetic, sympathetic, and central nervous system. In tantric language, we refer to them as Ganga, Yamuna and Saraswati, the three holy rivers of India.
According to yogic understanding, Ham and Ksham, or ida and pingala, represent heat and cold at the three levels of our being. Sometimes the brain works very fast and becomes overheated, or it may be under-heated and move very slowly. Accordingly we say that the person is either suffering from nervous tension or depression.
Imbalance of heat and cold at any level results in disease. Therefore in hatha yoga we have certain cleansing techniques, such as kunjal, neti, vastra dhauti, shankhaprakshalana etc. These were originally devised in order to balance the hot and cold elements, first of all, at the physical level, and then at the emotional and mental levels, because they are all interrelated.
These cleansing kriyas of hatha yoga are very important for the sincere spiritual aspirant. They are not to be regarded as physical or gross practices, but equally respected amongst all the yogic sadhanas.
By the practice of hatha yoga, one can even develop siddhi. There are yogis and saints who have walked on water and flown through the air. Definitely it is possible. By purifying the body, you can make it as light as cotton. This does not mean that the body will be reduced in size and proportion. But through the internal cleansing kriyas the entire system is unloaded and freed of blockages and toxic build ups. This is how the physical body becomes lighter, purer and freer. You know the story of Padmapada, one of the disciples of Adi Shankara who walked upon the water. He was a great yogi and bhakta. Of course, it is said that he attained this, siddhi through his unlimited faith and devotion, but it is also true that he achieved it through the power of yoga.
You also can do the same through the internal purificatory practices of hatha yoga. Not for the sake of show or magic or for attaining siddhi, but for the attainment of paramsiddhi, the ultimate power. Drop by drop, you can fill the ocean. In the same way, first you have to acquire all the other siddhis before you can achieve the ultimate siddhi, the power of enlightenment.
So, don't doubt all these things. Definitely, as yoga aspirants, you should have faith in this, and not only faith, you must have practical experience. When you have already seen that merely by one asana, say by shashankasana, you can get rid of your asthma, or your depression, or by one pranayama like brahmari, you can get rid of your hypertension or your anxiety, why don't you believe that these siddhis can be acquired by yogic techniques. So one should not doubt the power of yoga. Always remember that anything and everything is possible. When it is possible to attain nirvana, through the power of yoga, all these other things are so insignificant.