Googling "pre-show work" does not turn up anything that resembles a definition. My assumption of what it literally sounds like would be anything that is done prior to the show. Obviously, with most magic tricks, there is work done before the show that goes into the tricks, like props or whatever. However, given the way the term was used in the post, it sounded like it had a specific connotation (slightly disparaging) that I am not familiar with.
I assumed the berglass effect was "safe" to discuss as I assume that nobody here, at least insofar as the version done in the video, actually knows for sure how it's accomplished.
Perhaps you could give me a hypothetical example that does not pertain to a specific "effect"?
