Sonic 2: Battle Race is a TWO PLAYER ONLY hack. Players race against each other on the same screen. If a player falls too far behind his opponent and disappears off the screen, he dies. The leading player then gets a point, the losing player reappears next to him, and the race continues. Whoever has the most points at the end of the level wins.
Pause + A: Quit the current level and return to the level select.
Pause + B: Kill both players and return to the last checkpoint. This is handy for "standoff" situations, where the players can't progress (they fell off a MTZ nut, for example) but they don't want to backtrack for fear of losing a point to their opponent.
Monitors work the same way as in split screen mode, but with an extra life giving you an extra point instead.
Knowing the level layout is important, particularly in the later levels.
All zones are playable except Sky Chase and Death Egg.
Edit: Here's a video of the hack in action, courtesy of Iceman404. It probably gives a better idea of what the hack's about than screenshots:
I realise that the vast majority of you were probably expecting to play a single player game and have no interest in a two player one, so if that's the case then I'm sorry for the disappointment.
However, if like me you've always wanted to play Sonic 2 against a friend on anything other than Emerald Hill, Casino Night and Mystic Cave then hopefully you'll have some fun with this hack.
All comments are welcome!