High-lift devices are incredibly important for the operation of aircraft. Particularly for large commercial jet aircraft, the use of flaps and slats is essential to allow acceptable take-off distances and land with reasonable speeds. They are incredibly powerful, which causes a massive problem if the flaps operate differently from each other, which we call flap/slat asymmetry.

This is possible for many reasons, but a common one is if a flap extension/retraction system jams, the two flap settings will become different from each other. Flap asymmetry is worse than slat asymmetry. Flaps provide a large increase in coefficient of lift and in drag for the same angle of attack as a clean wing, whereas slats increase drag slightly, but delay the stall greatly, allowing the aircraft to fly at a much higher angle of attack. Therefore, a flap asymmetry will cause a large rolling moment away from the extend flaps. and a strong yaw towards the side of the extended flaps.

Therefore, to counteract this as pilots, we must roll towards the extended side, and yaw away from the extended side.

On this flap indicator, our right-hand flaps are at flap 15 extension, so must have jammed, and our left-hand flaps are retracting through flap 5/flap 2. We must therefore roll to the right and yaw to the left to maintain straight and level flight.

This is a flap display off the Boeing 737 and actually indicates the location of flaps and slats, if you know the order of flap extension/retraction, but the examiner wants to keep things simple by saying it is only trailing-edge flaps.