Since it's on camera, I'll comment on that first because it's an intentional red herring. This is a live effect, and the camera is hard to fool in illusions, but as we all know it can be used to advantage. There are two advantages here:
- First you cannot get up close to the people in the video to examine them closely. More on that below.
- Second, here the camera is used craftily to imply to anyone who knows visual effects that a split screen effect has been set up. For instance the camera moves only vertically at the beginning, and the vertical wall is the obvious area to split the screen and composite a different picture on each side. This is not what is happening though, it is an obvious red herring. BUT it is probably what is happening near the end during the interview sequence where random people are going up and down the stairs.
Note in the early part of the vide, there is a TIME LAG between when a person walks around the corner of the stairs and when he or she appears on the other stairwell. This is because it is not a visual effect, but a physical effect. There is NO TIME LAG in this effect during the interview at the end. This implies the interview segment is a visual effect with a split screen on the vertical wall separating the stairwells. The editor in his or her mind is thinking that action must match properly and so matches the timing so that it is more or less seamless.
Now the actual effect, which does not need camera processes to succeed:
- Twins. Many illusionists are twins, and many employ twins to appear to be in two places at once, or to travel impossible distances. Here, the illusionist himself is a twin and he uses two sets of twins to reinforce the effect: the girl in the glasses and green top, and the man he "meets" again at the bottom of the stairs. The time lag I mentioned above is a giveaway that the effect is not a camera effect; it is merely the closest timing the performers could get by listening to each other's movements or hearing other cues. This is also why the effect works on camera better, because some people can tell even close twins apart when they get near. On camera, this is harder.
- So here, one twin walks up the stairs, the other comes up from the lower stairs. One goes down, the other appears above. You leave one on one landing, the other is standing on the other landing.
- Conspirators. Obviously the twins and camera person are in on it. As well as every one of the people holding hands in the later part of the video. There may be more.
- (sorry... it's that simple).
Sooo... don't trust your own eyes is not the moral in these illusions. Don't trust your assumptions and suppositions is the moral...