Welp, the idea is negative wheel friction causes them to accelerate instead of slowing when they touch the ground, unless they are at rest, so one of those videos shows him shoot a car and then it starts flying around.
PS Here's my crazy weird video of the day, it's like the Epic of dumb TF2 GMOD videos. Engineer has to
kill i mean rescue everybody when the evil cartoon from an an alternate universe makes them all into robot slaves.
http://www.youtube.com/watch?v=dzMq5_thk4o(just in case that part got confusing)