I suspect they use some cleverly constructed patterns in sprite tiles and then change the sprite attributes after each scanline during horizontal blanking. edi

I suspect they use some cleverly constructed patterns in sprite tiles and then change the sprite attributes after each scanline during horizontal blanking. edit: nope, the video from the sibling comment says they just used one of the background layers. BTW I tried running some of the games form that video on the GameBoy emulator I made and it didn't end well. It turns out all PPU timings are important, who would've thought :P