Sega Master System / Mark III / Game Gear
During the intro, hold +++. Wait for Tails to blink twice - you should hear ring sounds. Release the buttons and press .
On Green Hills Zone act 1, when you go up on the first loop, there's a life monitor there. If you're fast enough because of the impulse that you took to climb on the loop, you will take the life monitor and pass directly. Press jump exactly on the diagonal green spring and try to return to where the extra life is, and it will be there again. You can repeat this as many times as you want.
See the main page for Sonic The Hedgehog 2 (ソニック・ザ・ヘッジホッグ２)