Since music exists in time and since time never moves backward, we must guard against loss of forward motion. Even during a diminuendo, there must still be forward momentum so the line will not be broken. Think of a train that approaches from a distance at fifty miles per hour (crescendo), passes by (peak), and continues on its way down the track (diminuendo), never slowing down as it recedes into the distance. Similarly, a diminuendo should also sustain its rate of forward motion as it recedes into silence.
How the last note of any phrase or composition is played has a profound effect on the listener. If the final pitch is support with intensity until its extinction, the feeling of forward motion will continue in the listener’s mind even after the note has faded to nothing. Sadly, it is true that the musical line can be destroyed even during the last note of a phrase or composition.
McGill, David. Sound in motion: A performer’s guide to greater musical expression. Bloomington, IN: Indiana University Press, 2007.