I cant confirm this with actual experience with THIS version of armor plating, but I can say that it LOOKS to me like that bottom pocket would be for the window to go down into. Previous versions of the doors had the window sliding back and forth on tracks, held in place by a spring loaded pin. The problem was that the pin very often dislodged itself or didnt engage, and the 50 pound block of window would come flying forward (usually cutting off or breaking the hand of the person sitting in the seat.)
Like I said, I dont have personal experience with the new version shown, but I DO have experience with the doors and stuff on the 1151s, and looking at these new doors, there is some kind of mechanism behind the windows where they would normally slide back... hope it helps!