This is a brand new synthesis method!
When two sine waves are ring modulated together, the result is two tones whose frequencies are equal to the sum and difference of the input frequencies.
This synth places a carrier oscillator at an arbitrary frequency high above the notes that you play. Each voice is generated by ring modulating this oscillator by a sine wave whose frequency is halfway between the carrier frequency and the intended fundamental. The resulting tone is filtered to dampen frequencies generated above the fundamental.
The artifacts generated while sweeping the frequency of the carrier are amusing and sometimes lovely.