From the Super Mario Wiki
Dash Peppers are fruits appearing in Super Mario Galaxy 2 along with the Blimp Fruit and the Bulb Berry. If Yoshi eats a Dash Pepper, he will turn red and gain the ability to run much faster than normal; when Yoshi obtains this powerup, he is known as Dash Yoshi. This allows Yoshi (and Mario) to run up steep hills, up vertical faces, and across water during a restricted period of time. While the power-up is in use, a siren is heard. In all Power Stars in Hightail Falls Galaxy and the Prankster Comet of Starshine Beach Galaxy, the music in the level speeds up, and a high-pitched jingle is added which is in tune with the music. It only happens in these two places due to the music playing at the time; it is a cover of Super Mario World's athletic theme, and it is the only track in the game which has a different version for the Dash Pepper.
It is used primarily in the Hightail Falls Galaxy, where it is introduced. It is also used in the Starshine Beach Galaxy and the Bowser's Galaxy Generator. Once the effects have worn off, if the player is still on a vertical face, or if they have hit a wall, which slows both Mario and Yoshi down by stopping them, both Mario and Yoshi will fall regardless of the state/direction they are facing.
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