Ok, I'll have a go, but I'm not 100% sure.
I would guess that the dynamic pressure of the wind hitting the inside of the sock is greater than the drop in static pressure.
Like when an aircraft flies upside down. The increase in pressure would still be produced on the underside of the wing, and the decrease would be on the topside. The only way level flight can be maintained is with a greater angle of attack. (By this I mean the pilot would push the control column forward.) The greater AoA uses dynamic pressure to produce lift.
Does that pass your test Mr. Jaguar? As I said, I'm not 100% sure, but...
