From what I have seen, gravity controls the spinning. Watching from the outside, it looks like the cars that spin the most have the weight more concentrated in one area (like an adult on one side and kids on the other). When you go up a hill, the heaviest rider may not be on the end of the car going down first, but since the car spins, once you start going down, the car spins so the heaviest part is at the bottom (I'm not sure if this makes sense, but it will as you are watching).