From the Super Mario Wiki
In this form, the Yoshi can break through rocks; some of them take more time to break than others, often resulting in a set pattern of rocks the Yoshi needs to break. Like all forms, Yoshi only has a limited amount of time before he reverts to normal and is sent back to the door; clocks scattered about the obstacle course provide the Yoshi with more time. Going through a ring will simply return the Yoshi to normal.
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