Having declared they had rushed through their own version of the F-duct, we can speculate how it might work. We know that the McLaren duct is vented into the cockpit around the drivers legs. Then it is their leg that closes the duct to feed the rear wing. This alters; the flow through the slot in the rear wing flap, stalling the wing, reducing drag and increasing top speed.
Sauber have already run with a vented rear wing, theirs uses the inlet on the front of the wing, to blow through a slot underneath the wing. This allows the rear wing to be steeper without stalling, for more downforce. Their new shark fin bulging with the duct moulded inside it, feeds into this same slot. they could be aiming to blow even more airflow through the slot or like McLaren alter the flow to stall the wing.
But the flow through McLarens slot is driver controlled, so Sauber need to find a way to ‘switch’ the flow on and off. This could be done purely by the airflow being overcome by the drag creating inside the tortuous duct and hence cutting off the flow above a certain airspeed. Or they have found a way to vent the duct into the cockpit.
In Saubers case the duct does not pass through the footwell of the cockpit as in McLarens case, so how might they enable the driver to seal the duct? The placement of the duct may be gives us a clue. It is possible that an opening exists within the side ofthe moncoque. Sited near both the ducts inlet and running accross the frotn of the sidepod to the side of the tub is the impact spar, this could lead to an opening into the cockpit and allow the drivers elbow to seal the duct and redirect the airflow. Its not normal for teams to want to create any opening in the side of the chassis to improve stiffness and crash protection. But it is possible.
Therefore the driver presses his elbow against the opening at high speed to achieve the same stall as McLarens drivers get with their leg.