From the Super Mario Wiki, the Mario encyclopedia
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The Luigi Block is a type of block that appears in the Wii U title, New Super Luigi U. Despite its name, it is modeled after Mario's "M" rather than Luigi's "L". The block appears at the beginning of levels, but only after Bowser is defeated. 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 (this is untrue for New Super Mario Bros. U Deluxe). The first time the player hits the block, a message will appear describing the block's effects.
If the player hits the Luigi 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.