It was mentioned as a means to compensate for parallax error, and that would be stupid.
You can look through a scope at the correct amount of eye relief and move your head a little to detect parallax error.
I think you can guarantee that any parallax error will affect accuracy at some point.
I don't understand why you would want to waste ammo determining something you can pick up easily just looking through the scope. By shooting you are compounding rifle accuracy (error) with parallax error.
Actually the opposite is true. By pulling your head back and centring the image within the black circle as John describes you eliminate the effect of parallax, so that rather than compounding the rifle accuracy with parallax you can isolate the rifle's inherent accuracy from the effect of parallax. IOW if your groups shrink when you do it, parallax may have been your problem. If your groups are unaffected, it likely wasn't.
I've done it myself. It is quick and simple and is one of those things that can help nail down the source of an accuracy issue. It also may help to shoot the smallest groups with a non-AO scope.
