We've established that Effuel is a fuel-saving device that helps your car run more efficiently. So, when you connect Effuel to the ECU of your car, it starts collecting data related to your driving habits. It doesn't instantly start making changes either as that wouldn't exactly be a good idea. Instead, it continues to collect data for at least 150 Km before it decides to make any changes. The Effuel device checks things like your acceleration and braking habits, your gear-changing patterns, and other things like the car's fuel consumption.