My understanding is that ground effect works when the ground or other surface interrupts the downdraft thereby reducing aerodynamic drag.
With cars, it seems that the explanations I've found describe using the car's body itself as a wing and creating a venturi between the bottom surface of the car and the track. However, with a car the aerofoil's "downdraft" is going upwards so there's no surface to interact with it. In this case there wouldn't be any ground effect.
Am I misunderstanding this? How does ground effect work in a car which is producing downforce rather than a aircraft where it produces additional lift?