When the voice produces a sound, many harmonics (or overtones) sound at the same time, and we normally hear this as a single tone. In overtone singing, the mouth filters out all harmonics but one, and the one that remains is amplified to become louder. This is then perceived as a separate tone, next to the fundamental.
In her video, Anna-Maria shows techniques that become increasingly advanced. She shows the overtone scale (steady fundamental, moving overtone), the undertone scale (steady overtone, moving fundamental), parallel movement and opposing movement of overtone and fundamental, and even complex compositions with two separate melody lines.
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