How can you make a magnet that turns on and off at the flick of a switch?

A normal fridge magnet is always magnetic; you simply cannot turn it off. But the giant magnets that lift whole cars in a scrapyard can grip a ton of metal, then let it all crash down in an instant when someone flicks a switch. On, and the metal jumps up. Off, and it clatters down. How can a magnet have an off switch?