The powered rear wheels are not on the conveyer, but the unpowered fronts are. They are unpowered, thus unable to rotate, therefore the conveyer is also stationary.
Because the powered rear wheels are straddling the conveyer (but not in contact with it), then they will either be supported on axle stands and thus the car will remain stationary, or if they are in contact with any kind of surface then friction would propel the car forward (provided that the rear wheels were fed power).
