…in order to leave all the Strengths to Nukva… Zeir Anpin gives Nukva his original back parts, which thus remain as levels of Nukva, whose government is based on Judgment. Instead of them, other new aspects are found in Zeir Anpin stemming from Kindness, and even his back parts – which Zeir Anpin receives anew from Abba – are in the category of Kindness.
Zeir Anpin and Nukva were initially joined together because in truth all these aspects belong to Nukva, which is made up of Strengths, except that before the Separation Zeir Anpin was also using some of them to serve as his back parts because his own powers of Kindness were not yet sufficiently repaired to be able to rule and function. This is the secret of the verse: “You forgot (תשי, TeShi) the Rock that gave birth to you” (Deuteronomy 32:18) – “His power weakened (תשש, TaShaSh) like a female” (Berachot 32a; cf. Rashi on Numbers 11:15).