dual mass flywheels are there to provide damping to the engine, to absorbe vibrations. you may think that moden engine are much smoother than older ones, but there not, in many cases, they are worse. the reason they feel much smoother is, amoung other things, is the engine damping.
look at an 'old fasioned' clutch. it has springs built into the clutch plate to dampen the transmition.
with newer engines, that are more hi tec, and produce more vibrations, these springs in the clutch where not enough. To fit enough springs in the central part of the clutch, the clutch would have to be alot larger, meaning the whole engine and trasmition would have to sit higher up in the car, giving a higher CoG: not good.
to solve this
problem, the DMF was originaly devolped by LUK, for BMW.
it consists of, as the name suggest 2 plates of metal which can move indepenantlry, held together with very long springs. A clutch plate that is fitted to a DMF, will have no springs in it, the damping is done by the flywheel not the clutch.
The springs become worn out for a number of reasons.
aggrassive pull aways, low speeds in high gears (when you can feel the car start so shake, its the springs bottoming out so to speak).
unfortuantaly, they are a high cost part, but they do just eventualy ware out.
the knocking you can here when you shut your engine off, is the first mass stopping, but because the springs are worn out, the second mass keeps going until it hits the stops, it then bounces back the other way and hits the stop at the other end, producing the knocking sound.
this in itself doesnt really cause any major problems, but it can reduce the life of the clutch. When replacing a clutch you should always check the condition of the flywheel, it may not be knocking yet, but it still could be worn out, in which case a replacment would be needed.
However, the springs are encased in grease. That grease can get out if the casing starts to brake up. a clutch covered in grease is no good to anyone!
On my LCR, the clutch slave cyl went, so the gearbox had to come out. i renewed the clutch and flywheel at the same time as they had both done 45k miles and showing signs of ware.
Some people have a thing against patten parts, but when in comes to a DMF, try to get hold of a LUK flywheel, any motorfactor should be able to
help (i can get you part numbers if youlike)
as i said LUK designed the things, and the one i took out of mine had LUK stamped all over it. if you buy from a dealer, you will get EXACTLY the same part, in a diffrent box, with a stupid price tag.