The deal is that you are not singing the notes that low using "normal" vocal technique, but rather by rattling (almost gargling) the back of your throat in a manner that has the same wave shape as a really low note. In essence you're "faking it", but when it's done well it sounds natural and deep.
It's not hard to get started on, but as with any technique it takes practice to get good at it. Here's how I was started on the higher-harmonic style (khoomei): can you do a decent Kermit the Frog voice impression? Try saying "Kermit" in Kermit's voice. Then just say the first syllable, "Ker", and hold it on the R, like "Kerrrrrrrrrrr". If you're getting his voice right your tongue will be in the correct position to form the small extra chamber in your mouth that creates the higher harmonic notes. Just feel the vibrations as you say "Kerrrrr" and adjust the shape of your mouth and the placement of your tongue until the resonance hits a strong peak. Again, the edge of your tongue is cupped against the roof of your mouth, creating a second chamber in which a resonant note can develop. The Khargyraa (low rumbling voice) is harder to learn, and it's best to have a teacher for that as it is possible to strain (injure) your vocal cords trying too hard to get it. It helps to drink some milk or honey tea or even a little olive oil, something to lubricate your esophagus.