This rule is one of the fun rules and reminds me to take it easy, the hard work is over. Unlike cramming for the final exam...there is no such thing in running...you bring what you have in you and nothing more to the test. So the key is to relax and rest.
I always tapper the last week before a marathon even if it is a training run. A marathon I use for a training run the taper will be the last four days prior to the event. During those four days, I may at most run 9 miles total. For my major race effort, that tapper will actually be the last 10 to 14 days.
I only started tapering for races back in 1994, but I noticed a big difference in my races. I discovered that I could postpone the wall for an additional 2 or 3 miles. In 1994, I ran the Mardi Gras Marathon in January and then the Marine Corp in October. My times at both events were identical 3:45. But in New Orleans, I actually ran the first 17 miles almost 5 minutes faster then the Marine Corp. My training was almost identical for both race but the only difference was I ran a few seconds slower per mile in the Marine Corp and I had tapered for the 10 days coming into the run. At the Marine Corp, the only wall, I found was the one I sat on when I changed out my shoes at the end. So now I'm a fan of tapering before the race. If it’s not doing something physical for you, I know it is doing something mental for you.