To perform a Jump Start, tap the Powerup button right before the countdown says "Go!" and your car will jump forward at the start of the race. It's a bit tricky ... timing is everything! Also if you push it more than once it won't work!
Note that the Powerup button depends on what controller you're using:
On touch screens, the Powerup button is the blue circle on the right of the screen.
On gamepads you need to be accelerating with the right trigger, and then press the A button. (Or the X button on PS4)
On keyboard, it's the SPACE key.
On Apple TV, it's the "Select" button -- aka push down on the touchpad.
Check out Matt's tutorial in one of our livestream videos here.