There are probably electrified tracks which are screwed down onto the rafters, similar to the way wooden risers* are put down for clay tiles to attach to. The tiles attach using metal clips or nails. SolarCity acquired Zep Solar a while back, who designed quick installation solar mounting systems. I assume they're using that team to design something fully integrated and fast to install. You'll probably just snap the tiles onto the rails after they are screwed (bolted?) down. The tracks would just plug into each other like Christmas lights.

They said they're producing plain tiles without panels inside for edge pieces and north/east/west roof faces.

* http://www.topnotchgenconstruction.com/wp-content/uploads/20...