As you are already aware, the substance that generally constitutes the partition can be described as hardness (kashyut), meaning something very hard that does not allow anything to penetrate its boundary at all. Likewise, the partition does not allow any trace of the supernal light to pass through it into Malkhut, which is the fourth level. It can thus be said that the total measure of light that can be enclothed in the vessels of Malkhut is blocked by the partition and rebuffed.
It has also been clarified that the five levels of opacity (ovyut) that are in the fourth level are incorporated and enter the partition, fusing with its attribute of hardness.29The opacity of the different levels of vessels are incorporated into the partition, adding a second quality, called opacity, to the partition in addition to the first quality of hardness. This is what the author of the Sulam referred to in section #20# when he explained that each of the five levels of receiving vessels are found in Malkhut.
Now the author is expanding on that point to describe how those levels of receiving that exist within Malkhut are incorporated into the partition after the first constriction (tzimtzum) that prevented Malkhut from receiving the light directly.