The effects in ground effect aren't "apparent", they're real. But difficult to exploit. There's a complicated explanation using a "virtual wing" and Prandtl circulation in my old aero book. I never did get too much into Prandtl.
I have used the bow-wave off semis and buses while cruising on my motorcycle. In just the right position, I could back off on the throttle, and not lose any speed, the bow-wave supplying a push.