This synthesis technique used by Casio originally in the 80s in the CZ line of synths. It is related to FM (frequency modulation), with enough differences to avoid problems with the patent used by Yamaha’s FM synths of the era. Intriguingly, it did a good job at mimicking many “analog” synth effects including the sound of a resonant filter.
Technically, the carrier (final output) oscillator uses a sine wave table, and that table is stepped through by a modulator that can speed up or slow down the playback of the sine wave. These two oscillators are always synchronized to start a new wave together; the modulator can be at an integer multiple of the carrier’s frequency. Since the carrier is never sped up so much that it starts a new waveform before the end of its normal cycle, it’s known as phase (the offset inside a cycle of a waveform) rather than frequency modulation, even though the modulator is indeed causing the carrier to “play” faster and slower than its normal overall speed.