From the Super Mario Wiki, the Mario encyclopedia
After initiating the attack, Mario and Luigi will run into the foreground. Luigi will take out a balloon and seesaw, with the bros jumping on each end of said seesaw. The player then has to press either or when Mario or Luigi are high in the air to make them pound down on the seesaw, at which point the other bro will be flung upwards and has to do likewise. The higher the point which the bros pound from, the bigger the balloon gets in the background, and eventually, it'll pop.
At this point, smaller balloons will start raining down from the sky and popping on top of enemies, doing damage for every balloon that pops.
For an Excellent Rating, the player must make sure the balloon is at its biggest when it pops. The balloon is the right size when it cannot entirely fit on the screen.
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