The "reason" behind it is because the magnets align the molecules in the fuel as they pass... My instant response to that was, "the molecules in the fuel aren't polar, and aren't effected by magnets".
They're literally 5x10cm sheets, less than 1mm thick, and they're under the bonnet, so that's pretty much negligible, but yeah that's what I said. Well, it provided a night of entertainment proving him wrong.
it's the "in" thing at the moment to strap magnets to your fuel line. However in tests it has proved to not make a blind bit of difference, apart from a physiological one.
meh.. I take it they didn't take into account that high frequencies, such as projected from a ccd chip (found in videocameras), will temporarily neutralise the polarity in fuel molecules. Hence magnets won't have any effect while using film equipment anywhere near it.. basic physics, d'ah!!