Wolfgang Saus demonstrates polyphonic overtone singing, aka throat singing, aka singing two melodies at the same time.
From the YouTube description:
Wolfgang Saus sings two melodies at the same time: bass & soprano of Pachelbel's Canon simultaneously. It's a short demonstration of polyphonic overtone singing skills (sometimes referred to as throat singing) used in special new classical compositions.
The interesting thing about doing this with overtone singing is: the melody was always hidden in the overtones of the bass voice. Many ancient composers intuitively created "harmonic" melodies out of overtones of a basso continuo.
Here's a neat visual presentation of polyphonic overtone singing: