Biggest air outlet I've seen for a long time... Hope it helps with reducing drag...
My guess would be that it's the rear tires. In an open wheel racing, tires are parachutes. In past years they created outwash with the front wing and brake ducts sending air through the front axle. With regs being what they are, you can't do much about the front wheels. Other teams are doing a better job of creating outwash to siphon air from the rear tires.