That is not a particularly good drawing and like drsmart indicated is the configuration you would find on a horse trailer or utility trailer. If you consider the "breakaway box" in the drawing as your house battery/batteries it becomes more clear.
On my 3402RL there are two wires going to the breakaway switch, one is blue which goes to the brake circuit in the pin box, the other is orange and black which connects to the "only" hot wire you should have in the box, likely black. This wire is connected to the plug and when hooked up has power from the TV, it is also hooked to the "non" fused side of the circuit breaker for your house battery supply to the unit. If it is in the correct place the battery cut off switch in the convenience center will not affect it, so it is live even when the battery cutoff is in the off position, as it should be.
Unless you are hooked to the TV there should be only one hot wire in the junction box on the pin, that is the wire from the TV and house battery where you get the 12V for the breakaway.
My daughters horse trailer has a small 12V battery (similar to a alarm battery) separate from the house batteries that supplies the breakaway as is indicated in the drawing you refer to.
Hope this explanation is not too confusing, it really is very basic as the breakaway is supposed to have power to it at all times and should it ever happen then the house batteries supply the 12V to the brakes.
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