I've been reading about the Mercedes f-duct. When the DRS is open on the
rear wing, a hole is revealed that channels air to the
front wing, improving the aerodynamics. Autosport has
a handily arrowed photo which shows the hole (and the fatness of the rear wing end plate required to let air flow through it).
As with the original McLaren f-duct, this is described as stalling the wing to reduce drag. I can see that changing the wing air flow would affect down force, but can anyone explain what "stalling" means here, and why drag is reduced?