I'm trying to understand exactly how this works and my mind can't grasp it. What does the spring do, and how is it held in place? Will it require me to weld the plate to the body?
You don't have to weld the plate to the body, but most do. You could bolt it on if you wanted to remove it later.
Basically, the button gets riveted to the fiberglass (or whatever) wheel cover. then you attach the backing plate to the car where needed. When you tighten the button, it becomes flush with the car. Very clean and simple. Depending on the strength of your wheel cover, I think 3-4 fasteners would do per side.
Just got back from a trip to Savannah, GA. This is the first long distance trip I have made in my Civic Hatchback since I installed the aero mods. Even while cruising at 75mph on the interstate, I was getting 62MPG. In the past, I would be getting around 44MPG while travelling at these speeds. Since I couldn't do a lot in the way of driving technique (other than a small amount of drafting) to improve mileage on a flat coastal plains interstate, this mileage improvement is purely the result of the aero mods I installed. Its interesting that although everybody is *****ing and moaning about the higher gas prices, this has yet to change their driving behavior and the traffic on I-95 was going just as fast as it always does.