My fire service mentor taught me this early in my career. I have also seen FDNY Capt. Mike Ciampo teach it as well. If you encounter zero visibility while searching on the fire ground and you have a Halligan bar, you should hold the fork end with the adz and pick end extended in front of you. If you come across a hole or edge, the adz/pick end will drop into it, the shaft will drop and push your hand to the ground, alerting you of the hazard. Small stuff to keep you safe.