The FCD (acronym for Fuel Cut Defencer) is any device that prevents the stock ECU from cutting fuel to the rear rotor when the boost exceeds a preset boost level.
This maximum safe boost level and behavior is coded into the firm ware in the engine management unit and can not be changed without reprogramming the ECU. The maximum allowed boost pressure is about 8.6 psi. The way an FCD defeats the fuel cut is by lying to the ECU about what the boost pressure is. The FCD is placed between the boost sensor and the ECU where it modifies the boost pressure signal by some amount so that as the boost pressure rises above the preset "safe limit", the ECU continues to see a signal that is below the limit.
There are a couple of immediate consequences to this fooling around. As the boost rises, the ECU must increase the amount of fuel being delivered to the engine in order to maintain safe and efficient operation. As the FCD starts lying to the ECU, the ECU will begin to under-compensate for the rise in pressure leading to a gradual leaning of the air-fuel mixture. The amount of error increases as the boost rises. For relatively small errors, the only penalty is efficiency. As the error gets larger, however, detonation becomes likely, exacerbated by the high boost pressures and accompanying high intake charge temperatures. Detonation under these conditions will quickly kill an engine. So, before we go any further, be forewarned that using an FCD and increasing boost pressure without also compensating for the ECU error with fuel enrichment (and preferably more efficient intercooling) can cause serious damage to your engine