The system appears to be connected to the engine air feed. I wonder if they're using the engine airbox pressure differential to switch it (or at least involve it in some way). There will be a big internal pressure difference in the engine airbox when at full chat compared to under braking. Perhaps the RB switches modes accordingly, or maybe they just dump the excess engine air (during overrun e.g. during braking) in to the wing system in order to reduce the effect of air box overspill on the rear wing and also help reduce CofP migration forwards under braking (resulting in a more stable car)