The way I understand it is that the swirl flaps restrict airflow at idle, causing vortices to mix up the fuel and air to smooth out the idle. That works fine when when they're new but...
...they're located inside the inlet manifold on the engine side of the EGR valve, which recirculates burnt exhaust gasses back into the engine for emissions reasons. The soot and partially burned diesel that the EGR valve pumps into the engine over the course of 100,000 or so miles (during warm up?) gums up the swirl flaps and makes them stick in their closed position.
This restricts the airflow into the engine so it won't rev, and because the electric motor that's trying to turn the shaft they're mounted on is quite torquey, if the flaps don't unstick the two part shaft distorts allowing one or more of the flaps to drop out and get sucked into the intake ports.
The plastic ones make a mess inside the intake port but the metal ones can jam a valve open which the piston will strike on its way up to TDC, resulting in a scrap engine.
I suffered the plastic flap failure on my Saab at pretty much the worst possible moment - February, at night, blizzard conditions, me, wife, teenage son and both elderly parents in the car and all on the way to East Midlands Airport to catch a plane to Tenerife to celebrate my Dad's 80th birthday - when the engine went into limp-home mode with twenty miles still to go.
The car wouldn't pull more than 15mph and a friendly trucker tailgated me all the way to the airport almost touching the back of my car and repeatedly blowing his klaxons. I guess he thought I was just driving like Miss Daisy because of the blizzard and should have gone faster, but I couldn't, I daren't stop in case the engine died and he couldn't overtake because of the heavy snow which was rather ironically swirling around us.
Apparently removing these flaps doesn't noticeably affect the idle, while engine performance and MPG are both significantly improved, especially if the EGR is blanked off at the same time.
Thanks for reminding me about this issue, it's made me realise that it's time I sorted it on my new future classic Alfa.
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