If I read this right, before the Bobert "improvement" the ignition is cut off if he is not on the seat, but he can still sit in the seat and mow backwards. After the improvement the ignition is not cut off if he is not in the seat - unless he engages both reverse gear and the blades.

Personally, I can see that being useful as a machine going backwards with the blades running with nobody aboard seems potentially hazardous.

Anyway, that makes it sound as if the dead man's bumhandle switch does two things - one is to disconnect one of the twin wires from the single wire (on which scenario the single wire would continue to the ignition) and the other that takes place at the same time is to connect one of the twin wires to the other of the twin wires, which then goes via a cutter deck safety switch to the ignition.   

At least I think that fits the facts. If I have the facts right.