Musar zu Schir haSchirim 3:7

Thus I have seen it written in the name of one great scholar that Dovid Hamelech was asking the Holy One Blessed is He to break and destroy Israel’s oppressors in the merit of the twelve tribes [shevatim] who were born to the four Matriarchs. For the acronym of the names of the Matriarchs is the word barzel — “iron”: Bilhah, Rochel, Zilpah and Leah. The merit of the Matriarchs stands by us to deliver us from the harsh judgments alluded to by the word “blood” because these derive from the female aspect of the Sitrah Acharah.. This is the evil Lilis and her entire camp, who incorporate within them the five shades of impure blood. The Sages rule in Niddah (30b): “the daughters of the Kusim have the status of menstruating women from the crib [arisah].” Arisah is a euphemism for Lilis’ camp because of its connection with the word eres (venom) — a reference to the venom of the primordial snake. Similarly, Amos (6:4) speaks of, “Those who lie upon beds of ivory and stretch themselves out upon their couches [arsosam]. The camp of the Shechinah, by contrast, is called a “bed,” as in, “Behold, Shlomo’s bed [mitah] has sixty warriors surrounding it” (Shir HaShirim 3:7). Since the “blood” mentioned above derives from the five shades of impure blood, which in turn derive from the evil Lilis, the female counterpart of the Samech Mem, the only way to weaken it is by invoking the female aspect of the realm of holiness, that is, the Matriarchs. For it is from them that the twelve tribes derive. In light of the above one must instruct the women to have in mind the merit of the Matriarchs — Bilhah, Rochel, Zilpah and Leah — as they put these metal pieces in place at the turning points of the year. For it is their merit that stands by us to deliver us from all evil decrees and it is a very praiseworthy that one’s deeds be accompanied by thought. This act of “unification” is of great significance and preciousness to the Holy One Blessed is He. Therefore it has the power to deliver a person from all evil and from all mishap, with the help of Hashem, God of Israel.