From the Super Mario Wiki
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“Hitting this block will change the character physics to match those of New Super Mario Bros. U. However, Nabbit's physics won't change. This effect will last until you exit the course.”
The Mario Block is a type of block that appears in the Wii U title, New Super Luigi U. In this game, the block appears at the beginning of levels, but only after players defeat Bowser. Hitting the block allows the player to switch to the physics of New Super Mario Bros. U, with lower jumps and better traction. It will not affect Nabbit and does not appear if Nabbit is used single player. The first time the player hits the block, a message will appear describing the block's effects.
If the player hits the Mario Block for the first time in a play session (subsequently triggering the message describing the block's effects) before the "time warning" sound effect finishes playing, the sound effect will start over after the player has finished reading the message.