When you're discussing something geeky very intensely.
When you know a lot of information about something unusual, usually nerdy, and express it with great enthusiasm.
Bruce: Batman never killed his villains until Robin died, which is the only reason why people began to call him 'The Dark Knight'. And in the series 'The Dark Knight Returns', Robin was a girl after Jason died.
Wayne: Dude, relax. I only said I was excited for The Dark Knight.
Bruce: Sorry, I was nerding out for a minute there.