It's when they show the Buffy musical episode on screen while onstage a group of actors mime the action. Plus, there's a person holding up posters prompting the audience and everyone sings along. And mocks Dawn relentlessly. It's a whopping good time.

Looking at your icon, I must say that Nathan Fillion was at the Con as well. I got to hear him in a Firefly panel, but didn't want to get in the hundreds of people long autograph line for him and Alan Tudyk.