As a graduate AE - not an authority on this subject, I have two observations. The image appears to have three rivets across the median of the end plate, so maybe the area above and below this line has been balanced to minimize the torque-effect of having one side much larger than the other. This is a mechanical issue.

Also, remember the top part of the wing contributes about half of the downforce - the vacuum created below the wing isn't affected by the cutout shown, so the effect of "spilling" a little high pressure from the top side is minimal. Leaking the flow in this manner might have a dampening effect and could reduce vibration, thereby reducing stress, thereby allowing a lighter construction (a speculation).

The answer to your question is to be found on p. 62 of the latest RACE TECH magazine, No. 49.  In short, the only time downforce is of interest in a race car is when it is turning and pulling high lateral G-forces.  This results in a yaw of 6-8° depending on the tires and other factors.  Solid endplates, in yaw, will blank off the critical inter-element gap between the main wing and the "flaps" (1 to 3 additional elements).  This will stall a part of the wing, depriving the car of significant downforce just when it is needed most.  This notch on the upwind side provides more additional lift than the same notch loses by spillage on the downwind side.  A quick look at the pressure distribution around a typical airfoil shows that far more than half of the lift is developed on the underside of an inverted wing.

Paul6

Is there a formular or percentage of "notch" as I'm looking at a wing for this application at the moment.

If possibe a scan of the article you mention would be very much appreciated, as w don't get it here in NZ.
Please send it via the llink for my address.

Cheers Dean