No, it has nothing to do with phase modulation, although it is interesting. Most sync inputs just reset the slave oscillator on the rising or falling edge of the input signal. This circuit feeds the sync input into a comparator. If you input (for example) a triangle wave into the sync, you can effectively generate an edge at any point of the waveform. As a result, the slave oscillator can be offset against the sync oscillator.CZ Rider wrote: There was the Aries AR-338 "PMS" oscillator that was based on the SSM2030 chip. They called this Phase Modulated Sync though and may not be exactly the same as PM, but did look interesting.
You can of course do this with any oscillator which has a sync input, if you have a spare comparator - input an audio frequency triangle wave and an LFO into the comparator, and the comparator output into the other oscillator sync input. You may want to mix the two oscillators to make the effect obvious.