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When you start the level, you should be staring at a four-wheeler sports vehicle. Run up to it and get on (wait! If you look past it and up a little, you'll see a small alcove which has a few items in it). Drive around the horse-shoe bend until you get near the dirty great big lump of land in the front of you (quite obviously a jump). Hold down your Dash key (usually / on PC keyboards) and hold down Action. After a few seconds, release the Dash key and let 'er rip!
You'll now be in an enclosed area of which you have to drive through and do jumps etc. I won't spoil it for you though!
Hit Roll (usually End on the keyboard) and Left or Right, once you have stopped.