Keith, I think someone fed you a line of crap. The rear O2 sensor is only there for one reason, to monitor the catalyst. The fuel injection control unit will not adjust the fuel trim based on rear 02 sensor readings. The front sensor fluctuates as the fuel mixtures goes through it's lean/rich cycles and if the cat is functioning properly the rear 02 sensor will read a fairly fixed voltage and will look like a flat line on the screen of an oscillioscope. In the event that the cat quits working, the rear sensor monitors the front sensor and a "catalyst efficiency" code is posted.

The later style O2 sensors, broad band sensors, and air fuel ratio sensors do, in fact, draw their reference air sample through the wire harness. The sensor needs ambient air to reference the exhaust sample against and this is why you get a high voltage during a rich mix and a low during lean. Because the difference between the O2 content in the two samples, the greater the difference the greater the voltage produced. Back in the day we used sensors that simply had vents on the back side of the metal casing but they tended to get plugged up with mud and debris and then the sensor was screwed. Strange as it sounds though, they actually pull that reference air in through the wire harness, between the actual wire and the plastic coating. Just remember that a low voltage signal means a lean mixture, low=lean.

Mark