Same analogy, inverted. Say your Xterra could run just fine upside-down. You've still got the wheels on top of the rack, but now the X is upside-down and the non-geared wheels are supporting its weight (they're well engineered). The free spinning wheels are again on a conveyor. Theres a solid, non-moving surface lowered to meet the actual drive tires atop the now-inverted Xterra with the same 1000 lbs as before. Again, this magic Xterra can run fine upside-down. Will it be able to move itself forward?