My brain woke up and realized this is a GZDoom feature. I'm only seeing it now because I prefer zdoom, and have never bothered trying it.
After some more experimentation it looks like it is scaling down existing values, so that the brightest sectors still shine enough to be visible in the low light areas. The Caves of Circe has some pretty unique lighting effects compared to the other levels, so you get to see how the lighting works better there.