Vocalist Anne-Maria Hefele demonstrates her remarkable overtone singing ability in this informative video. Overtone singing is a voice technique where it seems like one person sings two notes at the same time.

This is achieved by manipulating the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out of the lips to produce a melody. The partials (fundamental and overtones) of a sound wave made by the human voice can be selectively amplified by changing the shape of the resonant cavities of the mouth, larynx and pharynx. [source]