The following short sections from the Zohar (II 135a-b) are recited in many congregations just before the Barchu prayer which commences the Shabbat Evening Prayer (Maariv). Rabbi Moshe Cordovero writes (in Siddur Tefillah l'Moshe) that although the additional Neshama enters a person towards the end of the Lecha Dodi hymn, it only begins to illuminate them when Barchu is recited. This section from the Zohar elucidates the preparations necessary for this to happen.
Just as they [the six supernal days, the six sefirot of Zeir Anpin in Atzilut] merge above in oneness [and are nullified within the Or Ein Sof in absolute unity], so she [the seventh sefira, malchut of Atzilut] unites [the six weekdays] below [in the lower worlds] in the mystery of oneness.
The days below are elevated to a higher level...
This comes about by illuminating them from above with a radiance of Or Ein Sof, as the Shabbat day commences. Consequently all the workdays, and indeed, all the work that was done during those workdays, also become elevated into oneness and united. The sparks of holiness that are imbedded within Creation and have been elevated by using them for Torah and mitzvot during the six workdays, now become elevated in the mystery of Oneness on the Shabbat day where no work is allowed; it is referred to as the mystery of oneness because it is a lower level of oneness than the oneness of the higher worlds, where G‑d's Oneness is not a mystery, but revealed. Through this lower level of unity, the lower worlds become elevated to the level of absolute unity, as in the higher worlds.
[She does this] to be with them above.
The days below are elevated to a higher level.
[Thus] the unity [of the lower worlds] parallels the unity [of the higher worlds]. (Siddur Tefillot Mikol Hashanah p. 168d) The Holy One blessed be He who is One above does not take His seat upon His Throne of Glory…
This is a metaphor for G‑d's revelation of Himself, as the verse (Psalms 113:6) states "Who lowers Himself to reveal Himself in the heavens and on earth", as understood by the Zohar I, p. 195a. This refers to His revelation in the lower worlds, which does not take place until…
…until she [malchut, the seventh day, rises up and] enters into the mystery of oneness [as the completion and consummation of Creation], similar to His, so that Oneness [in the lower worlds] corresponds to Oneness [in the higher worlds]. This, as we have stated (Zohar II 134a), is the esoteric meaning of the verse: "G‑d is One [prior to Creation] and His Name [the life-force which brings all of Creation into being and sustains it] is One" (Zachariah 14:9).
The explanation of this follows:
The mystery of Shabbat is this: On the Shabbat she [the seventh day, and the seventh sefira, malchut (Or Yakar)] is united within the mystery of Oneness, so that the [supernal] mystery of Oneness [of the higher worlds] may rest upon her. [This takes place during] the Evening Prayer of the Shabbat eve, for then the holy Throne of Glory [i.e. the life-force which brings all of creation into being and sustains it] merges into the mystery of Oneness, and is ready for the holy transcendent King to rest upon it [i.e., for the transcendent revelation of G‑d to be illuminate the lower worlds]. Her countenance is irradiated with a supernal light.
As the Shabbat arrives, she merges into Oneness, and is separated from the "other side" [the side of unholiness], and all strict judgments are severed from her. She remains in unity with the holy light, and crowns herself with many crowns for the holy King. Then all powers of negativity [literally, "wrath, anger," a metaphor for the negative forces of the "other side"] and all adversaries flee from her and vanish, and no other power reigns in any of the worlds. Her countenance is irradiated with a supernal light.
During the week the countenance of Arich Anpin illuminates malchut. Since the darkness of the mundane world is so intense, it can only be illuminated by this lofty level. However, when the world is elevated by the onset of the Shabbat the sparks of holiness that have been elevated by the mitzvot done during the week stimulate an even deeper radiance to shine forth from Arich Anpin and illuminate malchut (Tefilot mikol HaShanah ad loc.) This illumination is the revelation of G‑d's Oneness, which is now revealed because all the powers of severity and negativity, which prevented her from being irradiated, have been removed. (Or Yakar)
And she crowns herself here below with the holy people, all of whom are crowned with new souls, which is the mystery of the additional soul [see Beitzah 16a] which every Jew acquires on the Shabbat.
Then the commencement of the prayer is to bless her with joy and radiant countenance and say, "Bless G‑d, who is blessed" (in Hebrew, "Barchu et Hashem hamevorach"). The word "et" [indicating something in addition to the main subject] referring to the Shabbat eve [which is also blessed]. "Blessed is G‑d, who is blessed" is that which elicits the blessings from the source of life, and the place from which all streams [i.e. the life-force which brings all the worlds into being, and illuminates all of the sefirot] issue forth to irrigate all things. And because it is the source, the mystery of the "sign" [the Shabbat is referred to as "an eternal sign" between G‑d and the Jewish People (Ex. 31:17)] it is called "blessed". It is the stream of the wellspring [the outflow of life-force from the Source Above into the worlds]. And since they [the blessings] reach there, they all [flow] for all eternity. And this is the meaning of "Blessed is G‑d who is blessed for all eternity".