Red Bull have been the last of the top teams to debut their f-duct. Like Ferrari they have had to engineer the set up into a chassis not designed to route the ducting through the cockpit. From this image from WRI2.net we can see the duct is far more complex than a simple tube from the cockpit to the rear wing. In Red Bulls case and similar to Ferraris solution, the duct is fed from the roll hoop and then goes into a split duct, one section leading to the rear wings flap and one which passes under the wing. This lower duct appears to be the default route for the air, before the driver engages the duct. Air simply passes under the rear wing for no effect. There is also a part of the circuit that routes into the cockpit to be closed off by the driver to control the airflow through the circuit. whats curious is that the duct appears to be linked to the airbox, this might be a simple mounting point or some influence from the positive pressure inside the airbox.
Red Bull have decided not to use their duct for this weekends race.