Mental practicing is the “other half” of practicing before a performance. It secures the piece in one’s memory and tightens the “nuts” and “bolts’ of technical spots. In traditional practicing, one relies upon the music, however the deep memorization at the unconciouss level is that which makes us feel secure. Being able to see and hear the performance with your eyes closed is the best way to acknowledge truthfully that you OWN IT!!
The famous concert pianist Arthur Rubenstien was known for his ability to look at a piece of music, then go and perform it without ever having played it on the piano. That is not what I am describing, as that is a fluke and very rare.
In one study, participants who mentally practiced a 5-finger sequence on an imaginary piano for two hours a day had the same neurological changes (and reduction in mistakes) as the participants who physically practiced the same passage on an actual piano.