This device is crucial when running boost on a Protege with a non-turbo ECU. A non-turbo Protege wants to run at a 14.7:1 air:fuel ratio, and you need to run much richer than that (around 12:1). Without this device, when you add fuel (be it by an FMU, extra injectors, etc) under closed loop, the stock ECU will see this, and learn to pull the fuel, making you run too lean under boost and detonate.

This device knows when you're on boost, and when you are, it will tell the ECU that you are running a 14.7:1 air:fuel ratio so no fuel is pulled by the ECU. This simple device can be the difference between a happy motor and a blown motor.