She is separating two sound vibrations in one voice. How is that even possible?
As a vocalist, hand to my heart, this is the creepiest thing I've ever heard.
I tried it in my room & my husband ran in because he thought I was dying.

Overtone singing is a voice technique where it seems like one person sings two notes at the same time. You can sing the overtone scale on one fundamental. Another fundamental has its own overtone scale, so in order to have more overtones to sing nice melodies, you can use different fundamentals and change them while singing.