Phase distortion synthesis
Casio's answer to FM: a cosine wave is read out at an uneven rate within each cycle, bending it into a sawtooth, a square, or something that sounds like a resonant filter.
An oscillator normally reads its waveform at a constant speed. Phase distortion varies that speed within the cycle — fast at first, then slow — squeezing a clean cosine into sharper shapes with more harmonics. Put an envelope on the amount of distortion and the tone opens and closes much like a filter sweep, without any filter being present.
Casio introduced the technique in 1984 with the CZ-101, a polyphonic digital synth with mini keys that ran on batteries and cost a fraction of a DX7. The maths sat close enough to FM synthesis to compete and far enough from Yamaha's patent to be legal. The CZ range sounds thinner and more "analog" than FM and is far easier to program, which made it the broke musician's digital synth of the eighties.