External relockers are located outside the lock body, but usually right next to it, inside the safe door. The external relocker stops the transfer bars from moving, so that the safe bolts cannot be forced back by a burglar into the unlocked position. Two variations of the most common arrangement are shown below.
If someone pounds on the dial enough so that the dial spindle drives the lock or lock cover backward, then it releases the spring-loaded relock bolt. On some safes (picture on right) it will block the cam from moving by dropping behind the cam or into a notch on the cam. Safes that use gear drives rather than cams will be different. On the gear drive Fort Knox on the left , the relock bolt goes behind a block welded to the transfer bar. Some safes have a notch in the transfer bar into which the relocker goes. A qualified safe technician will be required to open a safe after an external relocker has been set off.
Interesting note: The safe on the right is a popular brand that we will not sell, and the relocker is not assembled properly. The lock cover plate is installed so that the roll pin rests on top of it, rather than being fit into a hole in the plate where it is supposed to be. I guess it takes too long to make the parts fit correctly.
When buying a safe make sure it has an internal relocker. Most cheap safes, especially Chinese made safes, do not have them.