It's crap.
The theory is that it can fool the ECM into increasing or decreasing the amount of fuel to make more power. Problem is that every car is going to be different in the amount of fuel it wants. Some cars might have a limited power boost (maybe 0.5HP), while others will loose power.
I have seen two kinds of this "mod":
1. Replace the coolant temp sender with a fixed resistor. This effectivly eliminates the coolant temp and makes the ECM think the car is at a constant temperature. Several obvious reasons as to why this won't work.
2. Put a resistor in series with the coolant temp sender. This has the effect of shifting the reading. In theory, you could slightly control the fuel mixture a little bit assuming you calculated the correct resistor value to add or subtract the proper amount of fuel. In practice, it's crap.
Also you can get the same resistor from any electronics store for about 2 cents.