A bump key can be used to open most standard pin tumbler locks.
Most locks that you use every day are pin tumbler locks. The insides of these locks are pretty simple: a series of springs pushing downwards, with small metal cylinders (“driver pins”) attached to those springs. On the other side of each driver pin is another cylinder, the “key pin.” The image below shows the driver pins in blue and the key pins in red.
When you insert a key into a lock, the ridges of the key push the key pins upwards, which pushes the driver pins upwards, which compresses the springs. If it’s the correct key, the top of all the key pins will line up so there’s a consistent gap all the way down the pins. That means the key can be turned.
If it’s the wrong key, the pins won’t line up and so you cannot turn the key.
People who pick locks typically try to move each pin up individually so that they all line up, but it turns out there’s a much easier way to do this: use a bump key.
A bump key is just a normal key of the same type as the lock, but with all of the teeth as low as they can go. Insert the key in the lock then hit it with a hammer or other heavy object. That bump causes all of the key pins to jump up at once. This pushes the driver pins up and compresses all of the springs. The key pins will of course fall right back into place, but there’s a small delay before the springs push the driver pins back into place. To put it another way, the bump pushes open a small gap between the key pins and driver pins. If you turn the key before that gap closes, the lock will open.
You can see this effect (in slow motion) here:
I gotta say, the fact that this lock picking technique is widely known – and has been for years – makes me feel just a little bit insecure. Locksmith companies have various mechanical fixes for the lock bumping problem involving more complex key shapes or pin configurations, so hopefully this technique will become less successful over time.