I went to the track last night and saw some fast cars. A few of them right before they launched, they sprayed a white smoke from out of thier hood. What exactly is this? I assumed it is NO2, but why would they shoot it into the air?

My guess was this...they open the bottle and too much builds up in the lines for the engine so they let some of it out of an opening in the line....is this right?

 

I guess the are purgeing the no2 system of unwanted air or so in the lines. my next door neighbor has some sort of purge valve hooked to a switch on the dash in his firebird and he does this all the time when using no2.

 

You are correct. They do this to get air or moisture out of the lines, so when they launch there is a pure charge of nitrous into to the motor, and to relieve pressure in the bottle, there is a certain amount of pressure that the bottles should not go above.