RVP is defined as the absolute vapor pressure exerted by a liquid at 100 degrees Fahrenheit. It is a common measure of the volatility of fuel. The higher the RVP the cleaner it burns and the better the fuel efficiency.
The ECO Fuel Enhancer increases the Reid Vapor Pressure in fuel (diesel, gasoline, natural gas, propane, bio-diesel, etc.) causing it to burn more volatile.
WikipediA Vapor pressure definition (click here)