Correct me if I'm wrong but I think the general idea, is that the chips trick the ECU into thinking the engine is taking more air in than it is, so it compensates by chucking loads more fuel in. The effect, you're now pumping way too much fuel into your engine, which will damage it after a while. Also I think the resistor chips are no more than resistor's you can buy from somewhere like Maplins for about 17p.