Sega Master System / Mark III / Game Gear
When Knuckles appears to steal the emeralds during the opening demo, hold and press when he is over the third emerald (taking it or not). A ring sound will confirm the code. Start the game and the Stage Select screen will appears.
See the main page for Sonic The Hedgehog Triple Trouble / Sonic & Tails 2 (ソニック＆テイルス２)