Lock bumping is a technique criminals use to break into locks. It involves inserting a special key called a bump key into the lock and then tapping or 'bumping' it with a hammer or other tool. The bump key has ridges that are the same shape and size as the pins inside a lock. When the key is tapped, it causes the pins to all move up at the same time. This lines the pins up with the shear line and allows the lock to open.