From the Super Mario Wiki
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A Spring Topman with its spring exposed.
A Spring Topman is a variant of Topman, appearing in Super Mario Galaxy. One first appears in the Buoy Base Galaxy near the area with the Water Shooters. It only appears when Mario/Luigi spins a wheel, causing it to appear. A Sentry Beam that looks and acts similar also appears at the top of the tower. It later appears as part of the Dreadnought Galaxy's Topman Tribe. It resembles a large green top. A Spring Topman can use a spin attack which does not damage Mario, but pushes him away, possibly into other hazards that may hurt him.
If Mario tries to stomp a Spring Topman or spins it, its head will pop up, immobilizing it, and a second stomp will spring Mario up for a jumping boost (similar to when Mario jumps on a Spindrift in Super Mario 64). Mario must take advantage of this to reach high ledges and items. The only way to defeat one is to spin it into an electric fence.
A Spring Topman in Super Mario Galaxy 2
The enemy later reappears in Super Mario Galaxy 2, and is found only in the Space Storm Galaxy.