ECU: Performance Chips
There is an old saying that say "Anything that sounds too good to be true probably is" Consider this when judging the quick and easy solutions to more power.
While some vehicles can be upgraded with just a computer chip, a 3S can not. These devices are not actually a chip anyway. All they are is a resistor. By installing the resistor between the ECU and the air temperature sensor, the ECU will be tricked into thinking the air temperature is lower then it actually is. As a result, the ECU advances the timing and leans out the air/fuel ratio. This could possibly yield one or two horsepower at best, but certainly nowhere near the claim of 20hp.
Also keep in mind that if you advance the timing too far, or lean out the fuel mixture too far on a Twin Turbo model, this can result in deadly engine pre-ignition (Knock)
Notes from Jeff Lucius
The above is not entirely accurate. Because if the inverse relationship in the intake air temperature (IAT) sensor (higher resistance means lower temperatures), adding another resistor inline in the circuit fools the ECU into thinking the air temperature is lower than it actually is. At both high and low air temperatures the ECU retards timing; timing is never advanced based on air temperature alone. See my web page http://stealth316.com/2-ignitionsystem.htm . However, if the air temperature is above about 133ºF (an approximate value established from DSM ECU programming - see the dsm-ecu yahoo! email group) then the ECU retards timing. In this case only, lowering the air temp can increase (advance) timing by 1 to 3 degrees by changing the ECU's timing retard for high temperatures back to 0 correction. Below ~133ºF timing will remain the same or be retarded by lowering the air temperature value.
In addition, artificially lowering the air temperature will make the ECU think the air is denser than it actually is. Denser air means higher engine loads, and higher engine loads mean reduced (retarded) timing.
In short, intalling a resistor to lower the IAT signal voltage *may* increase timing by 1 to 3 degres only if the IAT is above ~133ºF, which is fairly uncommon when a 3S car is moving except in the hottest of weather. When IAT is below ~133ºF then timing will remain the same or be retarded. At all temperatures, the cooler IAT will fool the ECU into think that engine load is higher than it actually is and so timing will remain the same or be retarded. [End notes from Jeff Lucius]