Take it you've seen the thing on ebay. Pretty expensive for a resistor...£4.99
It fools the ecu into thinking the engine is colder than it currently is..so it richens up the mixture.
I remeber trying this on my old uno turbo - the only benefit i noticed was a slightly smoother pickup on low revs, and a drastically reduced mpg. It didn't make one difference, and I wouldn't want to try it on a car with a cat...i can see the engine not burning all the fuel as the mixture would be way to rich.
Also with this mod you are supposed to modify the air flow meter, to allow more air to flow at the previous intervals.
Personally, i think its a load of bolx.
Websites are like ********s, everyone has one, and im wary of any bloke trying to direct people into their's