When I hear a performance that is particularly moving, I know the musicians have been successful in creating and expressing their musical image. You don't sense the effort in what they are doing, only the result. Musical clarity is the product of well-defined choices. Call it imagery, visualization, hearing the music first, etc. What you call it is not important, the result is the same.
Mental imagery allows the musician to match their performance to the music in their minds.
This mental focus seems to transcend mere physical results. A study conducted by the Soviets prior to the 1980 Lake Placid Olympics provided some interesting results. There were four groups of athletes training for the Olympics. The fourth group, which had trained most heavily in mental preparation and visualization exercises, showed better improvement than the other three groups. By practicing of course, but not in the traditional sense.