is taking up volume in the combustion chamber, resulting in a smaller air/fuel mixture and therefore overall less fuel burned. Kind of like a crude way of decreasing the displacement of the engine. However, the effect of this would likely be negligible, and probably not noticeable.
More likely, the reason people on fuel-saver.org are noticing better fuel economy is that they are probably driving more conservatively. That will make a bigger difference than any of these devices.
Charles Carbon Black '95 TT **sold** Carbon Black '03 BMW M5 '02 Honda CR-V dogmobile