From the Super Mario Wiki
The Chest Station is a location in Bowser's body, as depicted during the events of Mario & Luigi: Bowser's Inside Story and its remake. This area is apparently near Bowser's heart. This area becomes active when Bowser crashes through Princess Peach's roof. He flies over to twin Piranha Plants. A trio of Fawfulcopters tell Bowser Fawful planted the fire breathing Piranha Plants. Starlow leads the Mario Brothers to the chest station. Here, Mario and Luigi use Bowser to fight off Piranha Plants over Peach's Castle. Later, they help him fight Dark Bowser. The player can use the stylus to swing Mario and Luigi in the air making Bowser jump. The farther the player holds the stylus, the higher Bowser will jump. If Chest Station is visited any other time, the cannon will not be accessible and therefore the area is rendered useless.
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