Between the second last and last beep hold down the accelerator (X). If timed correctly, this will give you a speed boost.
Full Turbo Bar
Hold the brakes and accelerate at the same time, do two 360s, then release brakes to get a full turbo bar.
Hold the brakes and accelerate at the same time. The kart should just stay in the same location. When the brakes are released the turbo will automatically activate.