This stunning demonstration of Tuvan throat singing is the result of tuning the resonances of the vocal tract. By creating a single low fundamental note with a rich harmonic spectrum, singers are able to enhance certain harmonics and minimize others. This gives the mistaken impression that more than one note is being sung at once. In this case, the two voices are also interfering, creating beats and difference tones. A particularly amazing demonstration of this starts at around 1:00.
If you’re having trouble hearing what I’m talking about in the context of the song here’s a more clinical version of the same difference tone phenomenon.
Thanks to Thor Magnusson for sharing the video.