When the time of the unification of Zeir Anpin and Malchut elevated to supernal Mother and Father, the four aspects of the Holy chariot awakened together with the soul of the righteous with all kind of singing and joy to welcome the daughter, which is Malchut. Their connection is in Holiness and it happen when the upper 3 Sefirot of Zeir Anpin join with Malchut in קדוש, קדוש, קדוש. That is 3 times ‘Holy’.
This state of holiness requires all the ten Sefirot to connect and draw the blessings for the unification.
At that time, King David, is awaken with the harp that plays by itself when the light of Chokmah from this connection connects to Malchut.
King David’s psalms has 10 different aspects to represent his connection from Malchut to all the Sefirot above. The harp that plays by itself is the effect of his desire for the light and higher connection.