Polyphonic Singing

Polyphonic singing, also known as throat singing, allows the musician to sing two pitches at the same time. The vocal chords make one sound, and the singer is able to create the perception of there being a second pitch through manipulating cavities in the larynx, pharynx, and mouth using an overtone that is produced by the vocal chords. You tune the spaces by changing how you shape your mouth, lips, or throat. In this kind of throat singing (Sygyt), your tongue moves behind your teeth around your gums, and a little hole is created behind your molars. You are then able to make it sound like you're whistling while singing because the sound moves between your teeth. The resultant noises can sound like whistles, growls, or even coughs.

http://www.youtube.com/watch?v=vC9Qh709gas