In case you want some pictures for reference of what I found out before digging into the file:
Imagine you have these three elements. A sphere, a plane and a glass plane
If you put the glass plane very very close to the white plane, you´ll be able to see (or refract) through it! See how you can see the sphere in there?
However, notice how although we can refract through the white plane, we can not light through it, light still hits the white plane and does not iluminate the sphere
This gif illustrates the whole effect.
You´ll see a similar example in the file provided.