As a concept goes, I completely understand how this works and the merits to stalling the front wing. What I'm unsure regarding is how the stall speed might be controlled. I am willing to postulate a theory based on varying venturi diameters in the front wing pylons which allow up to a predetermined CFM at a 'X' (mph) intake speed velocity which in turn stalls the wing or further stalls the wing which leads into the next theory. The obvious we can be sure of which is Mercedes don't want is the wing stalling at 100mph when they are at a track which has 125mph turns.
The other theory is the wing is in a state of constant stall but at varying rates based on speed. This makes me wonder if the wing were heavily raked at the front that downforce would be in a constant state of diminishing returns based on speed. Or it could be a combination of both.