said by Subaru:Only thing in his video I assume the breaker was disconnected? He did not really say much about how from the breaker panel it was connected.
You would want to install the outlet with the breaker off obviously. All that this setup is doing is making a normally hard wired furnace into a corded appliance no different than a coffee maker, vacuum cleaner, or TV. It's also installing an outlet next to the furnace which it will normally run off of.
When a power outage happens, you just unplug the furnace from the wall socket that has no power, into an extension cord that's hooked up to a generator. No worries about transfer switches, interlocks, back feeding the grid, etc.
This setup also makes it handy as a quick disconnect for the furnace but still gives you an outlet nearby for a vacuum, a light, etc to work on a furnace should the need arise.