Explanation from their website http://team6.mpgproduct.com/

Here's an easy way to explain MPG-CAPS™ to first time customers. Ask them if they know how a non-stick frying pan works. They'll be familiar with the way the coating keeps stuff from sticking to the surface of the pan. That's how MPG-CAPS™ work too, by coating the combustion chamber of your engine with sacrificial catalytic coating. This micro-thin coating acts to catalyze pump fuels and dramatically improve your performance. As long as you continue to use the MPG-CAP™, the catalyst will continue to inhibit detrimental effects of sulfur molecules and other impurities found in pump fuels, retarding the formation of sludge, varnish and acids that are harmful to your engine. Your engine runs smoother, cleaner has more pep and you get the benefit of better gas mileage.

MPG-CAPS™ will actually help your engine by decreasing wear on combustion chamber components and as an added benefit it helps your engine burn cleaner, reducing harmful emissions.

NB: I'm not selling the product but would really want to know if it the product is technically plausible enough to worth the try.