Stuntman: Ignition

You will most likely get this achievement when trying to complete all movie scenes.
If not, just try to "miss" a ramp on purpose by hitting it with only two wheels and steer into the direction you want to fall, or use a corkscrew ramp and stop spinning halfway in the air and land on your back.
If it still gives you trouble go to the Freestyle arena in the construction area and place a loop in the arena. Drive onto it and stop when you are at the top. You will drop down and unlock the achievement.

Go in arena creator and place a loop. Drive up the loop in any vehicle (except bikes) and brake in the middle (top) you will fall and land on your back.