From the Super Mario Wiki
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Ball Yoshi is one of the transformations from Yoshi Topsy-Turvy, and first appears in Tale of the Spirit of Speed. Usually at the start of a page, Yoshi may transform into a bouncy ball that cannot be harmed. This form is controlled purely by tilting the console, and upon impact destroys blocks and enemies alike. In the level 4-4, this form is used throughout the entire level.
"Destroy Everything! The ball's invincible! Don't panic as you tilt the Game Boy Advance with each bounce."