Reshimu comes from roshem, the mark, the imprint. It is one of the most discreet words of Etz Chaim and one of the deepest. The principle is stated in the gate of the Akudim: even when the lights rise back toward their source, "they do not withdraw entirely." They leave, in the place where they first stood, "a little radiance," and that radiance "is never uprooted from there." This remaining radiance is called reshimu. Chaim Vital links it to the verse of the Song of Songs: "Set me as a seal upon your heart." The seal is removed, but the wax keeps its shape.
The simplest image is the perfume flask: empty, it still smells. Or the room where someone has spoken: the voice has fallen silent, but something of the presence remains in those who heard. The reshimu is this presence of the absent.
Why does it matter so much? Because it is from the reshimu that the vessels are born. Etz Chaim explains it in the gate "Mati velo mati," the light that touches and does not touch: when the supernal light withdrew toward its source, the trace left below found itself without the light that had produced it; and through this distancing, "that remaining light became the reality of a vessel." In other words, a vessel is not a matter foreign to light: it is light thickened by lack, a light that remembers and which, for that reason, can now contain. Here again the withdrawal is fruitful, as in the tzimtzum: what remains after the departure becomes a capacity to receive.
The text adds a precise order. When Keter rises, it leaves a reshimu in its vessel to illuminate Chokhmah below; when Chokhmah rises, it leaves its own for Binah, and so on down to Yesod. Each degree receives from the degree above not its whole light, but its trace, and it is this trace that feeds it. Only Malkhut, Etz Chaim specifies, leaves no trace of its own: it is "the mirror that does not shine" and "has nothing of its own"; it lives on what Yesod leaves it.
For your study, you can keep two teachings. The first: nothing that has been given disappears completely; a blessing received leaves in you an imprint that keeps carrying. The second: limits, vessels, forms are not the enemy of light; they are its memory and its receptacle.
