it doesn't do anything at all. it doesn't smooth out the power or change it. it wont add 10rwkw or 1rwkw. it simply clamps the afm voltage when it goes near its peak value.
what happens is as you driving the airflow meter shows to the ecu how much air is coming in. when the value gets high on certain rpm levels (say near 4.5v, max is 5v) the ecu goes hmmm too much air is coming in, something must be wrong so it activates the rich and retard protection, to protect the engine. the result is a flat feeling and probably pops and farts out the exhaust. the ecu dumps timing and richens itself up to all buggery in a hope to make sure the engine is safe.
the fcd or "fuel cut defender" sits in between the afm and the ecu and says ok when i see 4.2v or higher (or something to that affect) ill actually tell the ecu its always 4.11v and won't tell it anymore that than.
so the engine runs along power comes on, air is coming in, goes to 4.11v and the fcd clamps the voltage and says to the stock ecu, hey its still 4.11v keep going, so the stock ecu does and thus you work around the stock rich and retard protection as the stock ecu never see's the higher afm voltages.
it wont magically add power or make the ecu think it can make more power.
there are similar devices like the apexi safc that do the same thing but they actually bend the afm signal to show less air is coming so that ecu goes ok so i see 3.1v of air but in actual fact there is really 4.2v of air so it uses more advanced timing and leaner fuel mixtures and as a result you make power.
there is no logical reason you would make more power by installing an FCD and nothing else. the only time you would see this sort of gain is if you are maxing hitting the rich and retard protection. and in this case the modifications that are on the engine, are causing it to make more power, as airflow makes power. the FCD is just tricking the engine to not activate its self protect mode