People have been running large shots of nitrous on LS1's for quite a while now. From what I understand* most of the time engines can handle a Nitrous shot of 50% of the engines HP. Figure LS1 + intake + Headers + exhaust = ~400HP.
Yes, he was purging the air from the Nitrous lines, and yes to prep the system for the run.
Nitrous 101
http://auto.howstuffworks.com/question259.htm
*This is may or may not be true, its just what i've seen on the internet.