A Crusher is a powerful obstacle found in Mario Kart Wii. They are only seen in Toad's Factory. They crush down on the conveyors periodically. Crushers are also the source of the Item Boxes found throughout the entire Mario Kart series. They appear to have a form of "juice" from an Item Box inside them and when they crash down on a Brick Block, they become an Item Box. The Item Boxes travel along the conveyors.
Crushers are usually found above the conveyors, the four on the left and right sides should generally be easy to avoid (unless you decide to get an Item) and the two in the middle are on the conveyor that move along the path. Being crushed by a Crusher is very dangerous as they can flatten you and cost you heaps of time. Running into a fallen crusher isn't much better as it can stop you cold. That's why it's usually best avoid them until you can time their movements.