From the Super Mario Wiki, the Mario encyclopedia
Yellow Springs are one of the obstacles that the Mini Marios encounter during the events of the Nintendo DS game Mario vs. Donkey Kong 2: March of the Minis. These objects are introduced in Mushroom Mayhem in Room 1-2. Their appearance is simply colored yellow with a flat board on top.
To use them, a Mini Mario has to approach it using the player's stylus depending on the direction the toy is going to make. When a Mini Mario jumps on these objects, they can use this spring to access to higher places than the toys would reach. These springs cannot move from its original place, but they are also not fixed onto the floor.
A similar spring introduced in the game is the Blue Spring.