The sensor is located at the top of the fuel tank. As the fuel level drops, the float also drops. This increases the resistance of the sensor, removed from the engine potentiometer sensor. As a result of this, the voltage supplied to the fuel gauge is reduced, and the indicator pointer is shifted in the direction "empty".
With further lowering of the float from a certain position, the contact of the warning device closes and a fuel reserve warning lamp lights up on the dashboard.