Just like vibrating strings, the human vocal chords produce audible overtones that correspond with the same harmonic series. Some of these pitches can be isolated and strengthened by changing the shape of the mouth, throat, and lips, and in Mongolia and Tuva in particular, this ability has been developed into a full fledged musical art-form. See this site for a primer on different types of “throat” or “overtone” singing. If you haven’t already, I recommend that you check out some recordings by groups like Huun-Huur-Tu and Alash (listen to this incredible performance). Khoomei is the basic style, while Sygyt produces a crystal clear whistling overtone, and Kargyraa is noteable for its production of undertones, pitches beneath the fundamental frequency.
It’s not exactly Tuvan throat singing, but do watch this scene from the classic film Baiju Bawra for a follow up to my previous post. First of all, it’s truly an amazing performance of a beautiful raag, Desi Todi, by two masters, Amir Khan and D. V. Paluskar. Secondly, be sure to watch the end to see Tansen and Baiju melt a slab of marble, break a tanpura string, and shatter a bowl of glass with the power of their voices!