If this links, the point of interest is the 'elevation' of the lower smoke filament passing under the XL1, and the 'elevation' of the actual diffuser panel at it's trailing edge.
You'll notice that VW doesn't expect the lowest streamline at the rear to 'recover' to an 'elevation' which is 'higher' than where it originated from.
The 'sink' is the same as the 'source.'
https://www.vwvortex.com/threads/vol...48569&slide=35