From the Super Mario Wiki
Spin Blocks are platforms that are found in New Super Mario Bros.. When stepped on, Mario or Luigi will start spinning and will fly high into the air, then glide back down slowly, allowing the Bro. to travel longer distances. Pressing the down button on the D-Pad while in the air causes Mario and Luigi to perform the Drill Spin, which has the ability to destroy Brick Blocks in a row or just to descend faster.
Spin Blocks are also found in the Nintendo Wi-Fi Connection 2-player mode. It only appears on the ice level, where a Star will eventually appear. There are however always coins hovering above the Spin Block.
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