The human psychic body has two types of nadis which as I know are Surya nadi and Chandra nadi. AFAIK, the nadis change automatically but one can also trigger them as needed. How to know which nadi is triggered at present and how to change it to other nadi?
Because this is a question related to physical practice, an actual teacher can best describe it. But I am providing a brief information so that it will be helpful.
There are actually many nadis, but three are important. Two you have already mentioned, the Surya nadi and Chandra nadi. They are also called as pingala and ida respectively. The third one is Sushanma (roughly the spinal chord). You can know which of the two nadis is active by this simple method:
Put your fingers under your nostril and exhaust air out of your nose so that you can feel it on your fingers.
You'll find that air from one nostril is coming in more quantity.
If the nostril is the right one, then Surya nadi is active and if the nostril is the left one, then Chandra nadi is active.
The rhythm changes throughout day from nostril to nostril and also sometimes both the nostrils are active. Regarding triggering them automatically, it's not easy. You have to practice yoga techniques like pranayama. If you can change or trigger the flow of air at will then you have already perfected pranayama which takes years of practice.
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