This segment from Bitwig's Polymer tutorial covers hard sync, a synthesis technique where a secondary oscillator resets the primary oscillator at a fixed rate. The effect is that the pitch control of the primary oscillator stops behaving like pitch and starts behaving like timbre, folding and reshaping the waveform each time it gets cut off mid-cycle.
The host demonstrates this on a sine wave first, showing how the waveform gets truncated at a regular interval to produce a more complex spectrum. Switching to a sawtooth, he then routes a per-note random modulator to the oscillator's frequency, so each new note lands on a different timbral color rather than a different pitch. This is the classic approach behind that distinctive Miami Vice-era synth sound.
Beyond the sub oscillator sync relationship, the sawtooth and pulse waveforms in Polymer also carry their own internal sync controls, which apply the same reset logic without needing a second oscillator in the signal path. That means you can layer the technique differently depending on which oscillators you're working with.