The term 'One Pulling Strings' refers to a person who is in control of a situation. This could be someone who is manipulating the situation to their advantage or someone who is simply in charge. In this case, the crossword solutions are two musical terms that refer to people who are in control of the music they are playing. A TUNER is someone who adjusts the pitch of a musical instrument to make sure it is in tune. A HARPIST is someone who plays the harp, an ancient stringed instrument. Both of these people are in control of the music they are playing and thus fit the description of 'One Pulling Strings'.