It’s all to do with creating vowels which have harsh clusters of harmonics that come together to create a second note on top of the normal note. So the unusual vowel that she’s making there (high, tense tongue and closed lips) create this strange, piercing sound, which she then fluctuates and controls with her lips. It’s really frigging tough to control – but if you imitate her you might be able to hear a slight ring of harmonics on there? I’ve been trying it all morning and only managed to get a tiny, tiny bit – this gal got game.