That's exactly right, it is a resistor. These types of things are quite popular for the BMW motorcycle crowd since the older oilheads tend to run lean from the factory and surge. They're installed not as a power booster but to mitigate the surging condition.
The website even acknowledges they are installed using two wires to the IAT (intake air temp) sensor. It's a NTC (negative temperature coefficient) resistor that increases in resistance as the air gets cooler. By installing an additional resistance inline to the sensor, you make the engine think it's cooler outside and it runs richer. With O2 sensors it does absolutely nothing since the ECU will adapt. And our engines don't run overly lean anyways.