1g Tech Article on Fuel Trims:
Fuel trims are used as a 'modifier' to the fuel map lookups to get the A/F Ratio at part throttle (closed loop) to where the ECU wants it. They are not used in open loop mode.
The ECU has a 'window' of adjustment. (Range of 60 to 140% for 91+ cars, 80 to 120% on 90's) If your trims are within the window (whether lean or rich, but not maxed), the ECU has corrected for any problems and should have an A/F ratio just as the ECU is expecting it to. Theoretically, as long as the trims are within the window, you really don't need to adjust your fuel delivery because the ECU is compensating.
If a trim is maxed, the ECU cannot adjust the fuel enough to fix the problem and you are running either leaner (FT maxed high) or richer (FT maxed low) than the ECU wants. This why Todd says he gets better gas mileage when his trims are maxed lean...because he is running leaner than the ECU wants. Leaner is less fuel. Less fuel used = more MPG (to a point...)
A dead or slow O2 sensor will wreak havoc on your trims. High values for all three trims (indicating a lean condition) are a good indication that your O2 is dead.
During idle and part throttle, low boost cruise, the O2 voltage should cycle up and down in the range of approximately .2v to .8v. If it doesn't, this would again indicate that it's time for a new O2 sensor.
An abnormally high Low Fuel Trim is a good indication that the heater in O2 sensor is dead. Most of the time, during idle, the O2 sensor will stop cycling after sitting for a minute or so. This is a good indication that the heater is indeed dead.
These are guidelines only. An improperly set fuel control, hacked MAF, boost leak, etc. can cause all kinds of results.
It is ok for the O2 to flatline at close to 0 volts during deceleration, even slight deceleration.
The Low/Med/High Trims we see are long term trims i.e. they are 'dampened' quite a bit (move slowly) and maintained between starts. They are derived from the trend observed from the O2 trim which is a short term trim (moves quickly). The O2 trim is derived from comparing the actual O2v to what the ECU thinks the O2 should be (stoichiometric). The idea is to keep the O2 trim right. If there is a trend (lean or rich) in the O2 trim for a period of time, it is corrected via the appropriate (high, med, low) trim. Which trim is adjusted is based on the current airflow. This will allow the O2 trim to go back to its midpoint because one of the other trims is making the fuel delivery correction.
O2 values and trims are ignored during open loop mode. The ECU uses a preset amount of fuel, ignoring any trim corrections. O2 Volts will not cycle in open loop mode. There are really no 'good' values for O2s at WOT because they differ between cars and sensors. .9 and above is generally considered pretty save. For some unknown reason, '90s seem to read .1v lower. I've run as low as .7x on pump gas on my 90. I would not recommend this without other instrumentation like an EGT.