by mhole » 9 Oct 2009 16:27
If you lift a pin to high, you can raise the key pin above the shear line. If you do this, and trap the pin with tension, you will appear to have set this pin, but will actually have trapped the key pin in the body of the lock. With the pin trapped like this, all you tension is being taken by the incorrectly set pin, and no other pins will bind, so you can't set them.
If the pin is correctly set, a tiny movement of the plug will trap the driver pin at the shear line, and cause another pin to bind up. When this pin is set, the plug moves a tiny bit more, and so on until the lock is picked. If the plug can't turn, you can't pick the lock.