If your talking about those resistor chips on ebay, those don't work.
Short story: It tells the PCM its colder out, dumps more fuel, car runs rich, and you lose hp. Unless your going to add more air, adding more fuel does not add more hp. Dyno proven to lose 20 whp on a Ford Ranger.
The end.
BTW, this is covered in the FAQs under snake oil. And FWIW, its nothing more than a 1 cent resistor you can pick up at radio shack.