From WikiP
Throat singers produce their harmonics through a process called Biofeedback. This means they raise and lower the fundamental frequency until they get maximum resonance on the harmonics sounding above, like moving a ladder up and down to achieve a desired height. They achieve this by controlling the manner in which the vocal folds open and close. When throat singing, the singer keeps the folds open for a shorter period and closed for longer. The abrupt closure puts greater energy into the upper harmonics, resulting in a clearer sound.
In addition to controlling the rate at which the vocal folds open and close, throat singers also manipulate the fundamental frequency through moving their jaws forward, and narrowing or protruding their lips.
As to actual pitch manipulation, there are four basic ways in which this is achieved. First, the tip of the tongue remains behind the teeth while the midtongue actually rises. The lips can also be opened slightly. This method is commonly used in the Sygyt style, discussed later. Secondly, moving the tongue forward can assist in manipulating pitch. The third method involves the throat itself. For lower harmonics, the base of the tongue moves to the rear of the throat. For mid to high harmonics, the base of the tongue moves forward until there is space in the vallecula, which is the space between the rear of the tongue and the epiglottis. Finally, a throat singer can manipulate pitch by simply widening the mouth in very precise increments, giving the effect of shortening the vocal tract. This is the easiest for one to experiment with and experience. The slightest opening or closing of the mouth can literally raise or lower a pitch.