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. This physics action can be visualized by observing the same effect on the desktop toy: Newton's Cradle. Because the pin movements are highly elastic, the driver pins "jump" from the key pins for a fraction of a second, moving higher than the cylinder (shear line of the tumbler), then are pushed normally back by the spring to sit against the key pins once again. Even though this separation only lasts a split second, if a light rotational force is continuously applied to the key during the slight impact, the cylinder will turn during the short separation time of the key and driver pins, and the lock can be opened while the driver pins are elevated above the keyway. Lock bumping takes only an instant to open the lock. The lock is not damaged in any way. Certain clicking and vibrating tools designed for bumping can also be used. These allow for rapid repetition of bumping against locks that have advertised "bump proof" features. Only a rare few key-pin locks cannot be bumped. Electronic locks that have a key backup are obviously completely susceptible to this method.

Nice description! I have a friend I was talking to about bumpkeys and bumping locks. She said she has bumped padlocks but not with a bumpkey....she was bumping padlocks with whatever key would fit it the lock. I didn't see it done but she sounded like she  knew what she was talking about. Interesting huh?

you can do this with some lock it depends on how the pins are set up, if you have the pins set in a high position it is posible that a key with ridges lower will bump the lock.

I was trying to practice bumping the door knob lock on my front door and having trouble.  Are there any tricks I'm missing?  How do I know how far out to start the key before I bump it?

The most common mistake when lock bumping is putting to much turning presure on the key, once the key with pulled out one notch (about 1/8 of an inch) you need to hold turning presure like you where using a tension wreach and hold the presure while you bump.