Lock bumping is a lock picking technique for opening a pin tumbler lock using a specially-crafted bump key. One bump key will work for all locks of the same type.
Bascially, a pin tumbler lock is composed of a series of spring-loaded stacks called pin stacks. Each pin stack is composed of two pins that are stacked on top of each other: the key pin, which touches the key when it is inserted, and the driver pin, which is spring driven. When the proper key is inserted into the lock, all of the key pins and driver pins align along the “shear line,” allowing the cylinder to turn. When the different length key pins are aligned at their tops by the insertion of the correspondingly cut key at their bases, the tops of the key pins and, consequently, the bases of the driver pins, form a straight line, so that the cylinder can be turned, rotating the key pins away from the driver pins. When no key or the wrong key is in the lock, pin misalignment prevents the cylinder from turning.
When bumping a lock, the key is initially inserted into the keyway one notch (pin) short of full insertion. Bumping the key inward forces it deeper into the keyway. The specially designed teeth of the bump key transmit a slight impact force to all of the bottom pins in the lock. The key pins transmit this force to the driver pins; the key pins stay in place.