"Fix dark glossy edges"

[Art48]

Hi,

What exactly is this parameter for? In my mind VRay should calculate every shader in a physically plausible way and there should be no need to "fix" anything. Am I wrong? Also why is it off by default. "Fixing" implies an error, and why wouldn't I have an error be fixed anyways?

[Svetlozar Draganov]

This option will fix dark edges that sometimes appear on objects with glossy materials.
The effect is very subtle - check the example bellow:

[Art48]

Okay and why does that happen exactly?