What does the ford fusion hybrid eco button do?

The ECO button in a Ford Fusion Hybrid will modify the throttle response at low speeds like when You are driving around town. You will have slightly less power in this mode, but the fuel system will also send less fuel to the engine. This can increase Your overall fuel economy and range if You need to go further. The ECO button will not do anything at higher speeds because Your car knows that You will need to get all the fuel You can if You need to accelerate to pass someone. The eco button will also not work in sport mode because they have different throttle mapping settings.