There might be a simpler way to explain this. Consider a vacuum cleaner. If it's running and you place your hand over the end of the nozzle, you can feel the vacuum go up (not just the drag on the motor). As you remove your hand, the vacuum goes down again. Your hand acts like the throttle in the carb - open the throttle and the built up vacuum has no resistance. Close the throttle (place your hand over the hose)and the vacuum goes back up again.
On my truck the wipers go like the band when I am up at cruising speed and let off the throttle, using the engine to brake. That is also when the blue smoke pours out...
Karl
