Came here to say this. It just cranks the brightness up for the pixels. But if you are already at 100% there is not much headroom so all the "before and after" for me were pretty hard to differentiate. There was some minor difference if I studied it closely but I'd probably not notice it during normal use it was so slight.

If I went down to say 50% brightness then it was much more pronounced, but who lives their life with their eyes only half open like that?!