To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated oxygen sensor. With extended switch points, the air/fuel mixture runs richer and leaner to overburden the catalytic converter. Once the test is started, the air/fuel mixture runs rich and lean and the O2S switches are counted. A switch is counted when an oxygen sensor signal goes from below the lean threshold to above the rich threshold. The number of Rear O2S switches is divided by the number of Front O2S switches to determine the switching ratio. The test runs for 20 seconds.
All, There is some really good info on what the PCM does and why it does it in the Onboard Diagnostics section of the service manual. Check page 25-37. Actually, the section on Monitored Systems is pretty good as well. That's on page 25-2. Here's something neat that i found...