 The diffuser "drives" the underbody, which produces the downforce.Front diffusers also exist (especially on Le Mans Prototypes or similar cars); however, they generate downforce purely from momentum exchange with the air, as there is nothing ahead of them to drive.A poorly designed front diffuser can create a low pressure region toward the front of the car which slows the air behind it down and reduces the effectiveness of the rest of the underbody.