An interesting one, this...
I remember as a child visiting Pleasurewood Hills Theme Park between Gt Yarmouth and Lowestoft and walking through a castle fo some kind. Within that castle, there was a topsy-turvy room where everything was on a slant and it was most confusing! It was all done through using familiar looking objects, but putting them at completely the wrong angle to fool the eye and body. Weird.
I imagine the illusion your teacher saw 30 years ago would work on the same principle of perspective altering - fooling the eye and thus fooling the brain.
I imagine that both the entertainer's floor and the floor beneath the spectators are sloped, at different angles, and that the table is built with false perspective in such a way that it fools the spectators into believing it's sloped one way, when, in fact, the opposite is true.
This is so difficult to explain without pictures!
Check out these
sidewalk drawings, and you might see what I'm trying to get at with regards to perspective - the drawings can only be viewed from one point otherwise they won't work. I imagine that this is the same for the illusion your teacher saw which is why there is a spectator 'walkway.'
I hope that helps and is not too confusing!