Piston

The piston is a machine that first appears in Yoshi's Story. In Mecha Castle, they appear in pairs and try to crush an unsuspecting Yoshi passing between. If a Yoshi is crushed, he will be defeated instantly and lost as a result. Even the Super Happy form cannot protect the Yoshi from this hazard.

A different-looking piston appears in Wario Land 4. If the piston squishes Wario, it transforms him into Flat Wario, allowing him to crawl into small spaces. They do not harm Wario or any enemy in any sort of way. These objects mainly appear in factory levels (such as The Curious Factory) and some Puzzle Rooms. There is always a Decompressor or a body of water that returns Wario to his normal size and shape.