Kevin Curtis, owner of the blog "The Accidental Yogi", has written an interesting yoga manual with an emphasis on the physiology of the various mudras, bandhas and kriyas. I particularly enjoyed his description of Kechari Mudra.
Khechari has a powerful stimulating effect on the deep brain structures. In fact, of all the physical techniques used in yoga Khechari mudra has the strongest influence over our brain's neurochemistry. The reason for Khechari's powerful influence over the brain is due to the direct stimulation the tongue has upon five of the cranial nerves and the erectile tissues in the nasal cavity - the stimulation of which is directly linked to left and right brain dominance (which plays an important roll in the awakening process).
By inserting the tongue into the nasopharyngeal cavity (Khechari Mudra) you are stretching and lightly contracting the tongue, which stimulates the vagus, facial, glossopharyngeal, and hypoglossal nerves, all of which lead directly to the deep brain structures responsible for the release of the hormones used in the awakening process. By pressing your tongue against the nasal septum (the divide between your left and right nostrils) you directly stimulate the nasopalatine and lingual branches of the trigeminal nerve, another nerve whose stimulation causes the activation of our deep brain structures.
With all of this deep brain stimulation and the resulting release of mind altering neurochemicals going on we can see why Khechari has been thought of, by yogis for millennia, as being the king of the mudras. This simple, yet admittedly difficult to achieve, placement of your tongue can have a deeply profound effect upon your brain and the way it functions, and consequently on the way you see the world and act within it. In the short term all of this change going on in your brain can manifest as emotional highs and lows that you might not even notice, though people around you probably will.
Noticing these types of changes is a bit like being in an airplane, when you're in the plane it barely feels like you're moving, but to people looking up at the plane from the ground it's quite obvious that you're moving at great speed.
With all of these dramatic, life-altering changes happening it's interesting and reassuring to note that the practice of Khechari mudra also induces a comforting sense of non-attachment - as if you're just watching the process unfold before you.
