What is the setting "Hand Bright Screen?"

    If you swipe down the watch face, then swipe to the left, you will see a quick access menu to do things like turn wifi and bluetooth on and off, turn on airplane mode, etc. There is a setting there called “Hand Bright Screen.”  Turning it on and off does not seem to do anything. Does anybody know what this is?

Raise your arm and turn it - the screen turns on.