The catalytic converted will convert more oxygen into carbon dioxide effectively making the exhaust more "rich". Narrowband O2 sensors like we have will read around .5v at stoich. As more oxygen is available due to running lean it will read lower voltage. If you remove the oxygen by adding more fuel it will read a higher voltage.
The PCM makes sure the cat is working by checking that the voltage from the secondary O2 is higher than the voltage from the primary. If you space the secondary O2 out of the exhaust stream a bit, it will have less exhaust pass over it and therefore come in contact with less oxygen and have a higher voltage than the primary and all is well. Too far and it won't get any oxygen and you will trip a code for a bad rear O2. I don't know if giving it a constant .55v will work since I've read the PCM still expects it to drop below .5v when the mixture goes lean.
JDMazdaFamilia, I am not sure where you got the idea that the PCM adds fuel to test the rear O2, but I have seen no evidence of it while monitoring my car, nor does it make any sense since a functional cat would already appear as a richer mixture to the rear O2. But I've been surprised before by this car, so maybe I missed something.