This relies on the principle of a discontinuity. A discontinuity in mathematics is simply a jump in some form from one value to another without any numbers in between. For example, if the temperature were to suddenly become 30 degrees Celsius this would be a discontinuity, whereas if the temperature were to change in reality it would probably be some intermediate value in between.
Imagine this: a black plank so long that it is infinite in length. Let us imagine that you are able to pass through the plank without it interacting with you, but that at the same time you are still able to see it. Let's imagine you are looking at that plank right now and that the plank itself is parallel to your body. You cannot really know how far the plank is away from you because that would require some point of reference which you do not have with an object that is infinite: you cannot find its centre.
Let us just say that the plank can be seen at an infinite distance. Look at the plank now: what do you see? You simply see a large, black, vertical line stretching upwards and downwards in front of your eyes: like a stripe masking a part of your vision. Let us now say that you are somehow able to rotate the plank: you rotate it and there is no change: the plank remains a black, vertical line in your vision. But something interesting is about to happen...
As you start to become closer and closer to being eye level with the plank you continue to see the plank in exactly the same way, but suddenly the black line disappears! Your vision is restored and the plank is gone! What has happened. The world stays like this for a while, as you stop the plank rotating and wonder where it has gone. You move very slightly upwards and the black stripe is back in your vision: you can see the entirety of the plank. There was no middle ground: you either had that black stripe in your vision or you didn't. The plank was either there or it wasn't and there was no evidence of it fading or coming back into existence.
