Again, some scope math stuff. It's oscilloscope synchronisation, that means that the waveform gets resetted each time the other not shown waveform is done. The C64 has this capability.
Interested in such stuff? Then check out the waveform studio. Wanna hear how that sounds? https://youtu.be/1OjPpVrc3gM (not completely sure) https://youtu.be/ty9DT87-QL0?t=92
