[BuildTheRobots]

My understanding is that they're glitching the game to inject data into memory, in this case processor instructions. And then they use another glitch to jump to and execute that code. No modifications needed to the hardware or ROM, you're just being extremely selective about how to play the game to cause memory to get flipped.

This video is done by hand and explains a bit more about how and why it works: https://www.youtube.com/watch?v=hB6eY73sLV0