Furnace disconnects sometimes incorporate both a fuse and a switch to disconnect power from a gas furnace. By unscrewing the fuse and/or shutting off the switch, you'll disconnect the power feeding the furnace. The fuse screws into the fuse socket and is covered by a metal lid that is hinged on one side.
The incoming feeder wire is connected to one side of the fuse. From there, the other side of the fuse is connected to one of the switch terminals. From the second terminal, the switch feeds the hot wire of the furnace.
Sometimes furnace disconnects are no more than a single-pole switch. If you ever have to do maintenance on the furnace components, you won't have to search for the main electrical panel in order to turn off a circuit breaker or unscrew a fuse. With this specialized disconnect switch, maintaining your furnace is a snap.