Originally Posted by Modshack

There are 2 fuel control systems. The first, when at idle or partial throttle, is controlled by the MAF and 02 sensors who are constantly sending readings back to the ECU to maintain ideal Stociometric ratios (14.7:1). This is called closed loop. An A/F gauge will bounce around a lot as the mix is read and constant corrections are sent to the ECU. When you go wide open throttle (open loop), the fueling defaults to some pre-built fuel maps in the ECU. These are usually slightly modified by the Long term fuel trims (correction factors) that are determined over time by the adjustments the 02 sensor Has been making. Typically the Z corrects from a slightly rich tune.
As Parts are added (Exhaust, Intake, HFC's), the rich correction is reduced as the Fuel trims approach Zero. Add Zero to the pre-built map and you'll have a bit more power due to the leaner mix.. (richer is slower). To truly see how closely you are to optimum, read the A/F ratio at WOT as is the case with all dyno plots.

In this picture, Additive is the short term fuel trims, Multiplicative is the long term trims: