It works off a MAP (manifold absolute pressure) sensor. This sensor reads the engine vacuum, and from that reading decides what the air/fuel ratio would be. On the other hand the MAF reads how much air is actually entering the engine.
With a MAP system, if you run something like a power adder, or larger cam the engine vacuum changes but the MAP is still looking for the vacuum of the original engine. So the MAP reads things wrong and the air/fuel ratio is also set wrong.