The reason you could not understand the illusion is that any explanation is false. It’s false, only an illusion.
“I don’t think so, because the people walking up the stairs experience the illusion.”
And David Copperfield’s assistants really believe he went through the Great Wall.
The people on the stairs are part of show. Watch the signs on the wall in the empty stairwell. Watch the tight shots of the people going up the stairs.
Camera trickery and not very well done at that!
What am I supposed to see when I watch the signs on the wall in the empty stairwell? They appear to be moving down as the camera pans up to following the person ascending the stairs. I do not see anything I would call a “tight shot” of the people walking up the stairs (and which stairs do you mean, anyway?).