The O2 sensor is like a battery and it sends a voltage signal to the ECU. The signal varies between 0 to 1 volts depending on the difference in the oxygen content of the exhaust and the oxygen content of the outside air. The ECU uses that information to vary the open/closed time of the frequency valve located at the fuel distributor. The frequency valve is kind of like an electronic injector but in this case it allows more or less fuel to be drawn away from the lower chambers of the differential pressure valves in the fuel distributor. The excess fuel is returned to the fuel tank. There is a diaphragm between the upper and lower chambers. More oxygen in the exhaust indicates a too lean condition in the air/fuel mixture before combustion. In this case the frequency valve will be open a little longer causing lower pressure in the lower chamber which allows more fuel into the upper chamber and thus more fuel is delivered to the injectors to richen the mixture. Less oxygen in the exhaust indicates a too rich condition. The frequency valve is opened for a shorter duration, the pressure in the lower chamber is higher and less fuel is delivered to the injectors.
This all happens at a very fast rate and is constantly changing. The idea is to achieve an ideal air/fuel ratio and it is done for emissions control.
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'80 DL 2 door, '89 DL Wagon