doesnt do really anything to the performance...you still have the same # of HP/TQ...it just determines where/when to shift based on your past behavior. Fuel trims are what the car uses to calculate the fuel needed for proper operation. The car starts out with both long and short fuel trims at zero from the factory. After the pcm calculates the amout of fuel needed, it injects that amount and then looks at the o2 sensor. If the o2 sensor tells the pcm that it didn't inject enough fuel the pcm adjusts the short term to compensate for the difference each injecton event until the o2 says it saw the right amount of fuel for what the pcm calculated. Once it does this it starts to adjust that into the long term fuel trim so that the short term stays as close to zero as possible.