Assume the Load is a Gas Valve. There’s more to the circuit than this, but it will serve the point I’m making. The ~ things are fuses in this example, but in the furnace, they’ll be switches that Open and Close in response to a switch activation, heat, pressure, or vacuum, or even Fusible Links, one shot deals that open permanently when too hot.
Furnaces have Door Switches mechanically operated, Temperature Operated Bi-Metallic Limit Swiches for Roll Out and High Limit Heat Exchanger Temperature, and Pressure Operated Forced Draft Switches.
With the Thermostat missing here, and integration into Circuit Boards missing too, it is usually more complex than this, but, in fact, the schematics often still depict a string of limit and proofing switches. Open any, and the Gas Valve shuts off.
Furnaces rely on these switches for safety purposes.
The Forced Draft Motor starts, but it has to be proven to have established a forced draft, so a pressure switch is used, it closes contacts to permit the gas valve control circuit to complete if everything else is OK.
If Roll-out occurs, i.e. flame from the burners enters the control wiring and gas valve area, the switches open (some may be a one shot deal, they open and stay open until replaced, or they may have plastic stems (usually red) on them, and can manually be reset).
If the Heat Exchanger is too hot, a Bi-Metallic switch opens until it cools.
If the Blower Door is removed, flue gases might be pulled from the chimney or flue and circulated into the home, so a switch opens and shuts off the gas if the Blower Door is removed or dislocated.
Watching furnace service videos on YouTube, and watching the guys that did the fall inspection on my furnace, and serviced it last year, I was surprised, when there’s a limit problem, that they remove the wires and check continuity rather than take voltage measurements.
Please see the Featured Image for the principles behind this Post.