The idea is this. Let's say you have a safe with an old style key (pre-yale keys). The shaft of the key would be hollow with an opening for a pin in the lock to slide into (not uncommon in old locks). But the pin is a key it's self. You turn the key, and key handle pops open to expose a combination dial (it could be a normal combination dial, but in my mind, I see three concentric rings so it would look more like a decoder ring). You would turn the rings to line up correctly, that would turn a rod back down the key shaft to actually unlock the safe.
The idea being that you have to unlock the key in the safe to unlock the combination dial(s) in the key to unlock the safe.