Piggy-back fuel controllers are popular because they allow users to modify stock fuel injection without replacing the entire ECU. They operate in various ways. Some of them modify the injector duty cycle control signals as they travel from the ECU to the injectors. Others modify input data to the ECU (like MAF for example), effectively “tricking” the ECU into delivering more or less fuel at a given RPM. Regardless of how exactly the piggy-back works, you’ll want to measure the results of any modifications you make to your fueling map.