It's the MIL Eliminator that you are talking about. It's just a resistor that gets placed in line in the wiring for the rear O2 sensor. It changes the reading so that the ECU thinks that there is still a cat in place. I have it in my car and have had no problems with it. The WRX doesn't use the rear O2 sensor for anything other then to monitor the emissions. The front O2 is the one that plays a part in open loop fuel tuning.