Interesting sound-effects can be obtained by the synchronising of (multiple) oscillators.
In a typical setup, one oscillator (master) restarts the cycle-period of the other one (slave) , what results in equal base frequencies on both oscillators. This is called Hard Sync.
The result is an irregular waveform with it's own harmonic spectrum, completely different from 'standard' waveforms
Soft Sync is a more general name for all kinds of oscillator synchronisation.
and distinctly different from hard-sync.
It is difficult to replicate this effect on digital synths, due to aliasing problems.
Soft Sync-like effects can also be created with other modules, for example a phaser or a Phase Locked Loop (PLL) Module.