CRTC/GA Vsync trick in 4-SINS demo

[slingshot]

Hi! Can somebody explain the trick used displaying 4 slices in this part of the Unique Megademo?
As I debugged, it splits the screen to 4x80 lines, and triggers a CRTC vsync at the end of each part (can be seen in emulators with black bars). I don't have the opportunity to test it on real HW, but I wonder how should it work there:
- blanking to black supposedly happen only during HSYNC, not VSYNC
- why only every 4th VSYNC starts a new frame on the display? As I see, emulators don't start a new frame until about 240 lines drawn, but in the GA decapped topic, I don't see an evidence for this in the hardware. Also there are other demos which are depend on this "VSYNC filtering". I must miss something about it.

[tronic]

https://open.amstrad.info/2020/01/12/analyse-4-sins-1997-n-w-c/

[slingshot]

Thanks!What I still don't get how the 4 vsyncs/frame from the CRTC does trigger a vertical retrace only after every 4th.