Hmm, sounds like somebody has been playing too much Shift...
Not really, I had no idea what "Shift" was until I googled it a few minutes ago...
how do you get down?
It already says how in the first post
If you jump high enough,you will flip over and you will drive on the sky. Then if you jump high enough onto a block on the ground, you will flip back over and you will drive on the ground.
It works by having two copies of the map, one is flipped, and is above the other. The "sky" is just a textured mesh you can jump through, and it takes you to the other copy of the map that is flipped over. Once you're driving on the sky, to get back down to the ground, you jump up to a box that you can hit from the sky. You go up through and invisible one-way mesh right below the box. This mesh has a physics driver, linear 50 0 -999999. That is fast enough to send you straight through the mesh, through the sky, and back down to the ground.