The method is called impressed current cathodic protection or ICCP. You may be more familiar with CP methods such as the placement of zinc anodes on to steel ship hulls. Instead of the hull material corroding away due to the electrochemical potential in the water (electrolyte), the zinc anodes push the potential of the hull more negative, and thus become sacrificial instead of the hull.
ICCP is a more active method by driving the potential, in this case the car's body metal, more negative using an impressed current. This is what this electronic box is supposed to do.
ICCP systems are used on pipelines and exposed metal pilings to inhibit corrosion. They are fairly complex systems, and are custom designed to their environment. They are used primarily when coatings or other passive corrosion controls are ineffective.
I suspect that this system is meant to be a fairly mild protection scheme, probably designed for the eastern Canadian market where you can get some fairly accelerated corrosion of body sheet metal. If you can get an extra year or two out of a new car before perforation, then it may be worth the 200.00.
Aggressive, or out of balance ICCP systems have been known to create problems in the material they intend to protect. Similar to what happens in your car battery, molecular hydrogen is created, and when absorbed by the material, causes hydrogen imbrittlement.
IMHO, a properly coated metal coupled with good maintenance practices should be good enough to keep your ride rust free.
UPDATE
Our chief engineer at work had a good chuckle over this topic around the coffee machine today. Turns out that around 20 years ago there was a company selling something similar, and was successfully sued in a US court for what amounted to false advertising, as these devices did not work at all. Something about the vehicle needing to be immersed in water continuously for this process to work.
Looks like Canadian Tire may have developed thier own brand of snake oil after all...
