When I want to use Nitro boost, I am allowed to do it only if the character looks forward. When I look behind the car, I cannot use it.
I also noticed that when I press the Nitro button and after that I look behind the car and then look forward and release the key, the consumption of Nitro does not stop - it continues until all Nitro is depleted. To stop this, I have to press the Nitro button again.
Video from the game:
http://www.youtube.com/watch?v=hlnx-Dic364