Accelerator damper

I have a 2000 Ford Focus Wagon [ Diesel]
I notice that when I take my foot of the accelerator pedal, the engine revs continue for appx two seconds, and then drop down to idle.
Seems to me there's a delay / damper in their somewhere. If so, what is it for, and why is that delay required on a diesel motor? John Hewitt, Malaga, Spain
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