It's explained in the threads a linked (someone proposed the idea, and the thread became a breakdown of how it could be implemented)
+ constant Movie (keystroke) recording
When you rewind, it loads a previous savestate and plays back the movie to the current time, capturing all the audio/video --- and outputting it in reverse order.
ZSNES would seem like it is simply auto-savestates, and pushing rewind simply loads the saves in most-recent order. (as back when I was watching lots of SMW hard hack Let's Plays, you could seem them repeatedly hitting rewind until they get to a position where Mario doesn't die before the player can react.)