You need the Massive Truck for this trick. Find the staircase with stone arc over it. Then, take a sharp left and climb the hill next to it. Next, cross the bridge and climb the path up the hill. Then, go down and you will see a little dune area. Immediately after you go all the way down the path, turn around and climb the little hill next to it. You may need to get some speed. After you climb it, carefully take the narrow path. After the narrow path, go straight and across onto the hill below it. Then, go straight towards the big hill but do not go down any hills. Take a left near the big hill and you will reach the hidden area. You will go very fast.