"Need for Speed: Shift" lets you use nitrous to get an extra boost during races. The amount of nitrous being used is displayed at the bottom of the screen for your reference. You can use this ability at any point during the race to maintain a lead or pull ahead of the competition. All version of the game offer nitrous, and each one requires the touch of a single button to use.
Press the "B" button to activate the nitrous on the Xbox 360 version.
Use the "Circle" button to activate the nitrous for PS3.
Press the "R" button to use nitrous in the PSP version.
Press the space bar to use nitrous for PC.