The rear O2 sensor is there to provide catalyst monitoring. The front sensor will flucuate up and down as the oxygen content in the exhaust gas goes up and down. As the exhaust goes through the cat the left over O2 is used up in the process and the amount that comes out the other end is pretty consistent so that the rear sensor will show little to no flucuations. If the rear sensor starts to bounce up and down like the front one then it indicates that the cat is falling off and it will post a catalyst efficiency code. Hope this answers your question.

Mark