When the diffuser is not in use, or rather when the rear seats are needed, the backrest it locks into place at the bottom near the seat squabs with either a mechanical or an electrically-actuated latch.
When the latch mechanism is released, the rear backrest pivots upwards around its top-mounted hinge until it is nearly level with the car’s beltline. In this mode, the rear seating area disappears just as it would with a normal wind deflector, but the backrest itself turns into the wind deflector. Judiciously-placed slots in the backrest are shaped to regulate the airflow through and around it, eliminating vibrations and reducing aerodynamic drag.
Best of all, the backrest can easily fold down again to restore the rear seating area without storing a bulky, loose wind deflector somewhere in the car.