This pops up from time to time -
What they do is sell you a resistor that you replace you IAT (intake air temperature) sensor with. It attempts to 'fool' the PCM in thinking it's cold outside, therefore adding more fuel. In reality, the car learns to ignore the sensor and often throws a CEL light.
By in the day long before the sophicated PCMs, removing the IAT sensor and place it in the fridge, after a couple of hours measure the resistance and then using a resistor of that value would be the simple way of over-fueling - not now.