From the Super Mario Wiki, the Mario encyclopedia
Spring Platforms are devices found in Donkey Kong Country: Tropical Freeze. They are composed of a broad wooden platform, sustained by two large springs underneath. They are only found buoying at the surface of the water, being held there by two pneumatic cushions.
The Kongs are able to stay on the platform normally, but they can also use it as a trampoline. Performing a Ground Pound action onto the platform will cause the springs underneath to compress, removing two side clamps that hold the platform in place, and bounce the characters high up. This allows them to access normally out-of-reach areas and items or grab onto Climbing Grass on the ceiling.
Spring Platforms are uncommon in the game, being only encountered in Mangrove Cove and Shipwreck Shore. Near the end of the latter, the Kongs find a special Spring Platform that will begin to raft across the water if Ground Pounded.