Display shadows using omnilight with negative checkbox

Godot Version

4.3

Question

Hey,
I want to display/highlight shadows using lights. I thought the “negative” option was exactly what I needed, but I don’t really understand what this button does.

This is a scene where I simply added a plain, a cube and an omnidirectional light (blue)


when I use the negative checkbox it changes the color instead of highlighting the shadow.

In another scene with the same setup except the light is white und the plain is green, it works as i would expect.
Without negative:


negative:

I don’t know if I just have no idea what the checkox is for or if I use it the wrong way.
If someone has any explanation I would really appreciate some help.
(The explanation in the Godot docs didn’t help me.)