It's not about the movie, it's the beer glass in the shape of a boot- if it's held with the toe pointing up, the vacuum holds the beer into the toe until air passes the curve at the top of the 'ankle' and then, it rushes out and soaks the person who didn't pay attention to at least one other person who did the same thing.
I Wanted to post a photo of someone doing it wrong, but YouTube doesn't have any videos that are easy to find......