Here is the quick explanation:

Lower screen is used during Veg and first 2-3 weeks of Flower to tuck under and weave (and at times to tie shoots LST style when they are too big to tuck and/or weave).

Upper screen is to use when Flower stretch begins to support weight of colas and keep them upright.

Last time I grew Black Domina the tops were so heavy they fell over and KO'ed other branches. I don't have the ability (access/space) to tie tops up to ceiling, etc since I'm growing in a closet. Hence the upper screen for support. And to make me less paraniod of falling colas that cause premature harvesting.

Hopefully this makes sense. If not post back and I will answer again to confuse the issue further. :stoned: