Dark grains on top to aid lautering?

In the recent issue of byo with the article on 10 dark clones there is a passage towards the end of the intro and before the actual recipes that states something to the effect (don't have it in front of me): "You may stir the darker grains on top of the mash to aid in lautering."

Why is this? I googled about but didn't really find any statements directly related to this. Some sources indicated that you may add darker grains towards the end of the mash if you prefer that flavor profile, but nothing seemed to indicate why it would directly impact lautering.

Stirring the top of the mash will help the lautering by closing any channels in the mash and forcing the sparge to flow through the entire mash evenly. But, dark on top? Maybe they are referring to the tendency of adjunts to stick the mash due to poor starch converstion. If you put them on top and stirred occasionally you'd avoid this part of the problem, but would the starch convert on the way through the pale?

That said, every brewery I've seen mixes the grains as completely as possible and stirs or rakes the top during lautering.

I was thinking it must be related somehow to adjunct conversion, but I'm not smart enough to draw any actual conclusions, and they hung that supposition out there without any background.

I don't have a mill, so I always mill my grains at the LHBS and typically let them fall into one paper sack (or two if they don't have the big ones...). I put water in the MLT and when it's the right temp I start dumping and stirring. They're pretty dang mixed by the time I put the lid one.

Maybe I'll e-mail a question to "The Wizard" and get printed in byo...