Here is an explanation using basic aerodynamic theory that shows that the wing does not actually 'stall' but appears to 'get smaller'.
Here is a normal aeroplane wing:
Air flows over the curved top surface of the wing as it moves forwards through the air. Because the air is being made to change direction the air molecules bash into each other as they bend around the curved wing. All this bashing around makes the molecules move further apart - they need more space to bash and move and bend. Moving further apart reduces the pressure of the air on the top of the wing. This reduction in pressure on top actually sucks the wing up.
The stall.
For this suction effect to work properly, the air needs to move around the wing surfaces and stay nice and close to the surface. If the direction of the airflow changes so that the flow is perpendicular rather than head on then obviously the wing will not work. It presents it's bottom surface and behaves like a fence panel being blown by the wind in a storm. Lots of force in the wrong direction = drag! This is a stall. A stall does reduce the downforce of a wing but it will INCREASE a wing's drag - NOT REDUCE it!
Here is a normal formula one wing:
See how the wing is angled to optimise the downforce but not cause a stall as described above? Notice also that it appears to be upside down? A formula one wing wants to push/suck down where an aircraft wing wants to push/suck up.
So, if the Mclaren rear wing device is not stalling the wing what is it doing? A non-stalled wing will create a force in the correct direction (down for F1) but in doing so it will also cause another force to act one the wing - drag. The wing creates this force as described earlier by creating a pressure differential - the high or ambient pressure on the flat side of the wing and the low pressure on the curved side of the wing. The Mclaren device essentially opens a channel through the wing to remove or reduce the pressure differential between the surfaces.
This reduction in the pressure differential will reduce the downforce but also reduce drag all without stalling the wing.
The super clever bit about this device is how it is controlled. No clever computers involved - just a knee! The driver opens or closes the channel by blocking a vent with his knee thus controlling the pressure differential over the rear wing.
Smart.
