Answer:
Nitrous Oxide is still used in drag racing but as far as i know it is not or has ever been used in NASCAR. Nitrous is a nonflammable gas that expands really fast when exposed to a heat source. It is used with the same idea as a blower or supercharger. Forcing as much air into the engine as possible. The engines have low compression pistons in them and the nitrous is injected into the intake usually in stages raising the shot pressure as the car moves down the track. Which in turn causes the fuel to burn allot easier, fuel is much more volatile under pressure. When you see the cars explode and they say it was a nitrous explosion it is almost the same effect as blowing up a paper bag and popping it only with fuel mixed with it.