As da Rock said, they're flaps kind of like some RC models have flaps for slow landings where they give extra lift to the wing but coupled directly to the control system. Because the stunt pattern calls for very sharp turns for some manoeuvres the normal wing loading simply runs out of lift and the model will stall in those turns. Deflecting the flaps far enough gives the extra lift needed to keep it away from a stall and fly smoothly through the corner.