The secret of Purim, as is known, is that during the entire period of the Exile, Nukva/Rachel stands back-to-back with Zeir Anpin.

At the end of the 70 years of the Babylonian Exile…the rectification process began in the supernal regions…

The partzuf of Nukva/Rachel can be understood to be the realm in which supernal divine sustenance is manifest. The ideal manner in which this holiness is communicated is via Zeir Anpin, known as the collective partzuf of the six intermediate sefirot, in a coupling known as "face to face" (which characterizes the Redemption, as well as other states of influx of divine mercy). This contrasts with the concept of "back-to-back", which is a kind of union that is greatly inferior to that of face-to-face. While the level of "communication" between Zeir Anpin and Nukva appears lacking in this state, the unification of back-to-back can be understood as a type of defense mechanism meant to ensure that Nukva/Rachel's most-often vulnerable back side is not in danger of "attack" by the External Forces, which are usually prevalent in her realm.

[The story of Purim] occurred at the end of the [divinely decreed] 70 years of the Babylonian Exile, as is known. At that point in time, the rectification process began in the supernal regions, whereby they would be returned [in a union of] face-to-face. [In order for this to come about] it was necessary that Zeir Anpin be rendered unconscious, meaning that Zeir Anpin's mochin leave [him] and enter Rachel, thus rectifying her partzuf. [As a result] she is then able to return [to Zeir Anpin] face-to-face.

Like Siamese twins joined at the back, the operation to separate Zeir Anpin and Nukva first entails the "anaesthetization", i.e. the departure of consciousness, of Zeir Anpin. Nukva then receives this consciousness indirectly, via Imma, which "enlightens" her, causing the External Forces to flee. The above union of "face-to-face" can only occur when the External Forces below are nullified to the extent that they cannot feed off, i.e. endanger, her.

The end of [this general process] of Zeir Anpin's being rendered unconscious occurred in the days of Mordechai. This is the secret of Haman's declaration: "There exists a nation scattered about and dispersed" (Esther 3:8) about which the Sages commented that Haman's declaration, "There exists [in Hebrew, "yeshno"] meant "Their god is sleeping [in Hebrew, 'yoshen', a play on words with 'yeshno', above] (Midrash Rabba Esther, 7:12 and Pirkei D'Rabbi Eliezer, section 50) when he said "…is sleeping", he was referring to their king - Zeir Anpin.

Like a king, which influences and provides for his nation, Zeir Anpin (usually) provides sustenance to Nukva.

This is referring to [Zeir Anpin's] state of unconsciousness, i.e. sleep, which was [really only in order to manifest his] desire to return face-to-face, i.e. to return and build the Second Temple. This is the meaning of "[On that night] the king could not sleep" (Esther 6:1) referring to the King of the world [Zeir Anpin] whose consciousness [mochin] returned to him [after the need for "anesthesia"] and he woke up.

[Translated and edited from Pri Etz Chaim, Shaar Purim, section 5.]