I agree with the friction part but wasn't expecting an increased ROD of about 300 ft a min or 15% mind you only done 2 in anger of about 5000 so mind wasn't really on the rpm's !
Nubian, they don't drop like a brick in a normal auto with engine running but at idle they come down at 1800ft per min hardly like a brick, if you want some real excitement ask Crab to do one in a Scout, that will be more like 3000 ft a minute
Pulling the fuel shut off valve would just shut it off instantaneously as you are pulling a lever. could try the mixture but initially the rpm would go up as you wind it out then would slowly starve the engine of fuel.