Need details on crank angle sensor & toothed wheel

[Pir0]

If it's anything like the Astra, the new aveo will have a thing, that, when you lift off the accelerator, because it's all electronic, the engine actually revs slightly, and the revs decrease slower. My guess was it's to stop that sudden thump of the engine returning to it's original place after lifting off quickly, to stop you being harsh on engine mounts etc from banging off and onto the clutch harshly while changing gears.