Multiphonics are an example of what technique

A multiphonic is an extended technique on a monophonic musical instrument (one which generally produces only one note at a time) in which several notes are produced at once. This includes wind, reed, and brass instruments, as well as the human voice. Multiphonic-like sounds on string instruments, both bowed and hammered, have also been called multiphonics, for lack of better terminology and scarcity of research.