If everything is set up the right way, no damage will happen. I've been using this stuff since the early 80's, and have had more fun than you can imagine. I currently run two stages on my Chevy with a total of roughly a 600hp boost.

Some very important things to running a safe nitrous tune up.

1. Retarding Timing
2. Plenty of additional fuel.
3. Start with the smallest tune up.
4. Depending on the tune up size, drop to a colder spark plug.

There are actually a ton of other things, but with a Cheater system I would start ith the smallest tune-up they suggest.