@ bonjon 1979,
That's pretty much what I thought. Cold air from the second air intake on the engine cover mixing with the hot exhaust gasses to provide a jet like flow to the rear of the car. Complicated, but once gotten to work can provide an advantage. Good to see that McLaren know you can come second at best by just following in the footsteps of others. They're trying to pioneer like they did last year with the F-duct, hope this time they can get a clear advantage out of it.
I always thought the second air intake on the engine cover wasn't for cooling(at least not for kers cooling). In 2009 they had kers in their car and no such cooling solutions were attempted.