Charting the flow
Picture the trailing edge of the wing in cross-section, with the Gurney pointing upwards and the airflow moving from left to right (above). At the trailing edge, the airflow immediately beneath the wing rolls into a small anti-clockwise vortex behind the Gurney. Immediately above this, a second small vortex, rotating in the opposite direction, is formed by the airflow travelling above the wing as it passes over the Gurney's lip.Together these two vortices form a small separation bubble — a rotating mass of air removed from the main flow — which is somewhat taller overall than the Gurney itself.
In clearing this separation bubble, the airflow's vertical deflection is increased and hence downforce increases. Additionally, separation of airflow from the wing's lower surface is postponed, allowing a higher angle of attack to be used before stall, which further enhances the wing's effectiveness..