Wikipedia on the subject of Lock Bumping

A lock is composed of a series of spring-loaded stacks of pins. Each pin stack is composed of two pins that are stacked on top of each other: the bottom pin, which touches the key when it is inserted, and the driver or top pin, which is spring driven. When the correct key is inserted into the lock, all of the key pins and driver pins align, allowing the cylinder to be turned. When no key or the wrong key is in the lock, the pin misalignment prevents the cylinder from being turned.

When lock bumping, the key is initially placed one notch out along the keyway. Bumping the key inward forces it deeper into the keyway. The specially designed teeth of the bump key impact all of the pins in the lock. The key pins transmit this force to the driver pins. 

Because the pin movements are highly elastic, the driver pins separate from the key pins for a split second and are then pushed back by the spring. Even though this separation only lasts a split second, if a light force is applied to the key, the cylinder can be turned and the lock can be opened.

The full text at Wikipedia can be found by clicking here.   

Some Links to "Bumping" videos.

 For your convenience these videos will open in a new window.  When finished just close window

This is a short introductory video to bump keys.  It is  three minutes long.  To watch click here.

A long but fascinating explanation to Lock Bumping.  This video is about an hour long.  To watch it Click Here