From the Super Mario Wiki
A Ground-Pound Switch is a type of switch that appears in Super Mario Galaxy and Super Mario Galaxy 2. To use this item, Mario must Ground Pound it (hence the name). Upon pressing this switch, a variety of things can happen, mainly making Star Bits or Coins appear.
There are also timed Ground-Pound Switches, tree stumps which have similar effects, and grey Ground-Pound Switches that make walls fall down, making other areas accessible.
They are also shown again in Super Mario Galaxy 2, having the same functions like in the first game. There is also a new function for Ground-Pound Switches in this game. If Mario Ground Pounds one of the switches, the screen would turn grayscale, and the environment and enemies on the respective planet would slow down greatly. This new function appeared in the Space Storm Galaxy and the Clockwork Ruins Galaxy, and was useful in avoiding stage hazards that would be impossible to dodge otherwise.
Also, Whomps have a similar switch on their backs. Activating the switch with a Ground Pound will defeat the creature.
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