Their diagram is showing that the fuse goes after the switch, nowhere near the battery. And also running 3 amps through a toggle switch. It's neat, but I don't think the people that designed really knew what they were doing with the electronic aspect (which is kind of important here).
Whether the device actually works, I'd almost wonder if it becomes more of a restriction than anything. It would be neat to see someone actually debunk this one to kill my curiousity. I wouldn't spend $50 on it though.
Pete