[Madact]

Apologies for the annoyed tone and partial off-topic-ness, but

More like a handy approximation of "the shape the air molecules want to follow for optimum return to the place where they were before you tried to ram them with your vehicle" given air flow that:
* is attached flow along the upper surface
* has a given amount of initial turbulence before you 'ram it with your vehicle'
* transitions to turbulent at a particular point (about 30% length I think, just looking at it?)
And the vehicle
* is close to a particular cross sectional shape
* is travelling within a particular speed range
* has a non ideal - but reasonably smooth - surface finish
In order to
* prevent premature flow separation (i.e. before a 'kamm' style cut-off) and
* reduce drag by achieving decent pressure recovery (given all the other assumptions and conditions)
Assuming
* form (pressure) drag is dominant over skin (friction) drag
* flow hasn't been tripped to turbulent too early
* flow can't be kept laminar much more than typical
* probably a bunch of other stuff I've missed here...

Sorry if I sound frustrated there, it just irks me a little that it keeps getting applied to everything, when the reality is a whole lot more complex. It's a great tool for vehicle modding which I wouldn't be surprised if it gets you within near-as-makes-no-difference of optimal when adding a kammback to your sedan, and a decent approximation of what you want the air to be doing in a general sense, but the "grand unified theory of fluid dynamics" it ain't.