Print screen doesn't work either?
I use the "block light" tool texture under some of my displacements to fix up some leaked light. Maybe they used that.
(I'd like to post a screenshot, but for some reason they all come out solid black)
What he's saying I would think is he doesn't want that sunlight bleed that you get in that image where it's closed and the light shines through due to the lightmaps.
btw, does it have to do with areaportals you think?