How do you jump start in BBR2?

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.