Uhmm, doesn't any wheeled vehicle roll down slopes without power?
Not necessarily if they have all certain friction values set up (see jeep)
Plus he's trying to make the sleigh powered, but slippery, or something. I can't follow anymore, I gave him a pretty big hint on how to do that.
Edit: see quote link for slippery stuff too
You'd also want to use setWheelPowered(wheelNumber,isPowered) and setWheelSteering(wheelNumber,steerAngle). You can find examples of these in the Tank script.