After you buy level 1, 2, or 3 hydraulics, come to a complete stop and hold R. Use the Analog stick to move up, down, left, or right. The longer you move back and forth, the more points you will get.

To activate your hydraulics, you hold the gas, the brake, and the hand brake. While holding those down, move the Analog-stick in the direction which you want the car to move.