www.yogimont.net/jia/overtonesinging/
Harmonic Singing Techniques In ordinary singing, the singer generally keeps the tongue flat and the fundamental is the only clearly audible pitch. To begin singing high whistle-like overtones, the sides of the tongue are curved upward and held against the upper premolar teeth, effectively producing two resonating chambers within the mouth. To try this yourself, sing "errrr." For higher overtones, move the tongue forward. Vowel sounds and lip shapes are important in fine-tuning the harmonics. The lowest harmonics are emphasized with tight "oo" sounds, while increasingly higher harmonics can be heard as vowels change through "oh...ah...ay...ee," and everything in between. While this will give one a basic starting point, we encourage you to listen to recordings of harmonic singers and throat singers and try imitating what you hear (hear 'em live too!). You will find with experience that more refined control down near the base of the tongue and in the throat allows one to amplify and isolate each desired harmonic overtone more effectively than just emphasizing tongue or vowel techniques. With the above in mind, a useful exercise is to sing each harmonic overtone through the series with your mouth completely closed (and keeping the fundamental as steady as possible).