Chris–I'll take a shot at it–it's kind of hard to describe, but it's a simple design. The Blackbird is designed for someone to sleep at an angle so that if they are lying on their back, their left shoulder would be off to the left of the center of the hammock and their feet would be off to the right of center. The footbox is some extra fabric that's sewn into that area where your feet go, and it allows you to lie much flatter. It also allows the integrated bug net to lie flat and not be stressed. Picture it as though you are lying in a slight V shape in a hammock, then someone came along and cut out a hole so your legs could stick out straight. Then they just sewed fabric to wrap around your leg's natural position and cover the hole. I can vouch for its comfort–it really is a nice design. I'm sorry I don't have a good pic.
Keep in mind that if you try to use a minimalist square tarp pitched diagonally so there are two corners tied out to the ground and the other two on the trees supporting the hammock, the footbox might stick out a little and there could be a coverage issue. With rectangular or hex tarps it shouldn't be an issue. Underquilts sometimes require some tweaking to work with the footbox too, but it's more of a set up adjustment than a problem.
