Nevil/Neil, I don't think it's quite as simple as just drying off the carpet. If you think about the amount of water you absorb into a bonnet compared to the amount of water that's in a carpet after cleaning, you have only removed a tiny % of it I reckon.
I think the secret is to wait the right amount of time, i.e. when the carpet is almost touch-dry. At this stage, a very light spray with something and a good bonneting over will catch any soil that has wicked up after the HWE and then with a good blast from airmovers you should halt any further wicking.
Very tricky to get right, but you can tell when you've got it right!