In Rally Mode, Lost Vegas Track, hold down the Left and Right Shift buttons, then look for the section of the track that is under construction. Goto the end of that sections, turn around, and keep going untill you hit a hidden jump.
For an offroad race, hold L + R while selecting any track except Rusty Springs.
To race in the Warrior Car, enter the Cheat Mode code, then exit tournament mode and hold L + R when selecting a car. Prepare yourself for a rough ride, as this baby accelerates at nearly 2G!