Via Los Angeles Times:
“Beatboxing,” as it came to be known, includes laryngeal lowering and lingual retraction, labial approximation, velic raising (to seal the nasopharynx off from the oral vocal tract, of course) and rapid raising of the tongue dorsal.
All this time, these artists thought they were just making beats. A team of linguistics and engineering students at USC, however, wanted to learn more about the mechanics behind the rhythms. So by using MRI technology, they recorded an unnamed local beatboxer working his magic, broke down the most commonly employed sounds by examining the movements of his mouth and then analyzed the data.