In some cases, imagining that you are doing something can improve your performance at the task as well as actually practicing it. The study used a task of identifying which of two lines is closest to a central line. After a baseline test, some practiced on a computer, and some just daydreamed about doing it. Both improved their performance.
I wonder first what kinds of learning this would work on. I would expect it to work with process learning: playing the piano, or learning a language, or running a maze. Not so much on history or biochemistry.
And then I wonder if mirror neurons are involved. These are neurons that fire either when you are doing something or when you watch somebody else do it; they are responsible for us crying at movies, being elated when our guys score in sports, and maybe for learning a first language.
And then I wonder if it had an influence in hunting and gathering. Did daydreaming about hunting caribou make Pleistocene hunters better at killing caribou? Or make it easier to find mongongo nuts when they were ripe?