While the D800 User Manual's description of this feature is very vague (as most user manuals are), I stumbled across this feature shortly after purchasing my D800. It's really quite a cool feature!
Here's how it works:
Let's say you've taken a picture of one or multiple people and want to get a closer look at their face(s) to make sure they didn't blink (or whatever). Doing it the old-fashioned way, you could zoom in and scroll to each face which, if you have several people in your shot, could take some time.
With Face Detection in Review mode, you magnify the image until you get one person's face filling up as much of the monitor as you like. You should now see the front command dial and face icons in the lower left of the monitor window.
Now comes the cool part: turn the front command dial one click at a time and notice how the monitor jumps from one face to the next, always centered. This lets you quickly review the faces of everyone in your shot! Cool, huh!?
Hope this helped!
_________________________________ The camera doesn't make a bit of difference. All of them can record what you are seeing. But, you have to SEE. Ernst Haas, 1985