[janzer]

My guess is the trick is needed four times per frame. For every field you need it once to start the static portion drawing, then once more to stop.

[anyfoo]

Ah, duh, you're completely right. I somehow hallucinated another buffer between PPU-generated frames and video output. Which would have been rather expensive at the time...

Also I only just learned that because the NES does not actually output the half scanlines that make interlacing work, both fields are drawn on top of each other, effectively making it 50/60 actual frames per second anyway, instead of interlaced fields! (https://wiki.nesdev.com/w/index.php/NTSC_video)