No no, you said that a ramp would cause it to jump downwards. Now how the hell is something launched off a ramp going to "jump downwards"?! The whole point of any ramp it to launch something in the air, upwards. This is simply achivied by the law of inertia and momentum, an object in motion won't change it's course unless acted upon by a force. If you launch a car off a ramp, it's moving in the direction of the ramp, the ramp's slope. It will continue that way till the end of the ramp, where it loses the ramp's force and is pulled down to earth by the force of gravity, 9.8m/s/s(it's the same for all objects and things in the world mind you). When driven off a ramp, the object will continue in the direction of the ramp, the slope, and then curve down to earth. The object does not just jump downwards, it continues upwards it can't overcome gravity's force.
And, if you look at the dune, as seen in the Motor Trend Wallpaper posted by Jordan, the launch part is kinda odd, it's flat. By common knowlage, dunes are shaped by the wind, which is not flat like shown in the pic. Also, the lack of sand in the air(there is a very small amount in the air, not the amount created by going over a normal dune) makes it seem like something was changed for the pic. While it might not be plywood or any wood or ramp, the dune could have been compressed by a leveller(the thing that vibrates with the flat bottom that makes sand, dirt, and other materials compact and solid, it's used in making driveways and paving roads) to create a surface stong enough to withstand the force of the H2 on the sand. This would also explain the hight and lack of sand in the air.
Unless I totally miss read your post, BMW, though I'm sure I didn't...