The mid wing area is very interesting, with the barge-boards the side facing the tires is obviously the high pressure side, but the side pod end plates, this is inverted, the side facing the car is the high pressure side. This causes air to favor the chassis facing side of the barge boards, as airflow tends to favor the path of least resistance, and the airflow to go on the outside of the chassis at the side pod end plates. This is further reinforced by the horizontal diffuser on the floor under the side pod end plates.
The barge board floor slots feed more air to the horizontal diffuser which then merges with the vortecies shed from the side pod end plates.
The low pressure at the leading edge of the floor will cause more airflow to favor traveling underneath the car, this strengthens the effect at the very rear of the car.
The side pods have massive undercuts to increase the pressure in that area, this increases the pressure differential and further boosts the effectiveness of this area. I have a feeling that teams learned last year how significant this area has become. Red Bull's big step forward last year came right as they began developing this 'mid-wing' area.