Great question. I think the answer might be something along these lines:
The detached outer parts of the sail are angled to reflect the light *inwards* and back at the central part, which means the force on the outer parts is both forwards and outwards to the sides. The outwards forces don't move the craft because the outer parts of the sail are attached to each other and are merely tensioned by the outwards forces.
The central parts of the sail push the craft directly backwards.
So, add up all the forces, and we get slightly more backwards force from the central part than forwards force from the outer parts, with the difference giving tension to the outer parts. The craft decelerates.
Someone please tell me (politely) if I've completely stuffed this up, as I'm no expert!