Pull up a Rocket, then pause the game before the Rocket takes you to another screen. Next, perform the suicide code (Up + B + A on Controller 2 while paused, then unpause). When you unpause, your character will have an empty power meter, but the Rocket will take you up to the next screen, explode, and drop you off as usual. Now, you'll be "small" with no dots in the power meter, and the music will turn off (if you're in World 4-1, you will have music because the music changes for the new area). Grab a Heart, and it'll fill in two dots, but you'll still be small! The next Heart you collect will fill in the third dot and make your character "big."