Shut up and watch.
Loosely translated: magic.
There are tutorials available online if you want to try and learn the spooky wailing. Just stay away from any passing exorcists.
Overtone singing, also known as overtone chanting, or harmonic singing, is a type of singing in which the singer manipulates 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. This resonant tuning allows the singer to create apparently more than one pitch at the same time (the fundamental and a selected overtone), while in effect still generating a single fundamental frequency with his/her vocal folds.
Loosely translated: magic.
There are tutorials available online if you want to try and learn the spooky wailing. Just stay away from any passing exorcists.