From the Super Mario Wiki
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Command Blocks are blocks in the Mario & Luigi series. They appear during battles, so Mario, Luigi, Baby Mario, Baby Luigi, and Paper Mario can jump under or Bowser can punch under the block that they want to choose. The block they choose determines the move they want to do.
In volume 35 of the Super Mario-Kun adapted from Mario & Luigi: Partners in Time, Luigi and Baby Luigi hit the Flee command block to run away from a Snoozorb, much to Mario's irritation, and therefore breaking the fourth wall.