From the Super Mario Wiki
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“The kingdom's latest fad! Launch Fit! It's easy! Just stand on the launching board! Oh, wait! There's something else. First, prep the launching board. The way you prep the board is... Oh, you know it, I'm sure! So stand on that board and... KABOOOOOM! Fitness explosion! Clench your abs as you soar! Unused muscles will tense up! Scared muscles flex like mad! It's Launch Fit! What a fun fad!”
Launch Fit, also known as Launch Diet, is a type of launching board that appears in Mario & Luigi: Bowser's Inside Story that can be used by Bowser to soar in the air like a cannonball, destroying anything in his path. To use it, Bowser must stand on the launching board and use his Fire Breath ability on a small bomb on the back of the board. When the fuse is lit, Bowser steps on the board, then the bomb explodes, making the board launch him across the area to the other side.