Background dim?

I remember (think) seeing a video a while back which showed that when you have a window selected, it darkens everything else on the screen. I'm wondering how you do this, or if it's even possible... it may have just been my imagination. (Not that I have a very good one)

If you don't know what I mean - basically;

• The window on top, which you have selected stays the same brightness, but everything else behind it which isn't selected dims out.