The short answer is that bump keys are common key blanks that have been cut down to the lowest depth across the blade, though the individual stations, or spaces, have been preserved by leaving a separator between each station.  This is simply accomplished by cutting each station with a code cutter, moving to the next, and then cutting the adjacent station in the same manner.  This leaves a 'hump' or 'point' between each successive station.  As the key is inserted, and also as it is withdrawn, the bottom pins are automatically bounced upward as the blade, with all these raised separators, travels beneath them.

With the added use of tapping (rapping) while the bump key is moved back and forth, and with a slight degree of turning tension applied simultaneously, it is possible to bounce the bottom pins enough that the percussive method of lock picking is brought into play.  This is roughly the same action that takes place when a pick gun is used, but it takes much more practice.

Bump keys, surprisingly, are so popular today that they are found on web sites that have little or nothing to do with the Locksmith trade.  It is likely that those who purchase these keys on a lark, with no real interest in learning how to pick locks as a viable part of becoming a Locksmith, soon loose interest because acquiring skill with them is no less difficult -- and perhaps more so -- than learning how to use "legimitate" lock picking tools.

We offer them because there IS a place for them in a Locksmith's tool kit, if only as a last resort.  Key bumping is more a hobby than a Locksmith procedure due to the simple fact that more skill is involved in conventional lock picking, and no Locksmith wants to be seen key bumping by a customer.