Swirl flaps are small butterfly valves located within the inlet manifold.
Designed to help regulate the fuel to air ratio and improve emissions at low engine speeds. At light engine loads the flaps close, causing the air to swirl into the combustion chamber. The swirling affect aids combustion and helps to generate a leaner air to fuel ratio. After 2000 rpm the flaps are generally activated to a fully open position and have a very poor effect on engine performance and emissions.

Due to their location within the inlet manifold any part of the flap or screw breaking away is likely to enter the combustion chamber. This often results in catastrophic engine damage, such as damaged pistons, injectors, cylinder head, valves, turbo etc...