[Daox]

As SVOboy said, you can lean out your air/fuel ratio and gain mileage. NOx emissions will go up from this. In Honda's lean burn engines they have special catalytic converters to deal with this.

However, that ad is total BS. Stock systems are already designed to operate at stoimetric levels and do it very well already. Leaning out a mixture will definitly not give you more power, it'll give you less. Richening up the mixture may provide sightly more power, but result in worse mileage. I depends on how the resistor is wired to the sensor as to what it'll do.