Red Switch Block
From the Super Mario Wiki
Red Switch Blocks are a type of block that appears in the game New Super Mario Bros. Wii. It appears only in the levels World 3-4 and World 3-5. When these blocks are first encountered, they appear as Dotted Line Blocks. To activate this block, the player must find the secret exit in World 3-4 and continue on to World 3-5. After completing these levels, the player has to stomp on a red ! Switch, which will activate the Red Switch Blocks. When activated, the Red Switch Blocks appear as red normal blocks. The main flagpole for level World 3-4 will then become accessible, and the player will be able to continue to World 3's castle. It is necessary for the player to activate the Red Switch Blocks in order to complete level World 3-4.