I never had it explained to me, but I just assumed it was to make sure the engine didn't stop...particularly important in something like the C150 with a Continental engine, as I've had it come close to stopping at that point a couple of times.
But I never thought to notice if the engine was idling too fast! Thanks, Genghis, now I know, I'm not likely to hit the hedge if I ever go to Netherthorpe...well, I might, but I won't have that as an excuse.
