Even though you can't normally use the toner and probe on a closed circuit, I still like to use the red led on my HP77/6A for continuity checks and to tell me when I've got the circuit open. Sometimes it's bad wiring or just light bulbs in a ceiling fixture with the wall switch in the on position or maybe a small appliance or table lamp plugged into a receptacle with its switch on. Once the toners red led light illuminates I can continue on probing the rest of the circuit.
