From the Super Mario Wiki
An Absorbing Mushroom is a special item found in Mario Kart Arcade GP. It is just a blue ordinary mushroom. When used, a color-vortex appears on the player as the player has to get an item from another player within the time limit. If the player hits another rival, the rival's item is transferred to the player. This item can be obtained when the player plays as Yoshi and wins a course.