Paul, I can't get into it in detail, but it's a feature called Fore-Aft Oxygen Sensor Control, or FAOSC. With a healthy pre-cat at a steady idle, the rear sensor reads about 0.6v, which is slightly rich. This makes sense because the cat brick is absorbing your excess O2. Now what happens if that point starts to drift, assuming in this case all the upstream fuel control pieces are working correctly? If it drifts lean, <=0.5v , it could either be the cat going bad, or a front O2 sensor shift. If it steadily drifts even more rich, >.6v , then the cat is obviously OK, so maybe the front O2 sensor is drifting rich. FAOSC logic isolates this shift, and then uses the rear O2 input to move the front O2 sensor lean/rich switch point back to stoichiometric.
