Restriction (Tzimtzum) is when I close my desire, disallowing myself to receive pleasure, like a baby who tightly squeezes his mouth shut and refuses to eat. A blow (Bitush) already assumes a condition: if I have the opportunity to eat for the mother’s pleasure, then I will happily open my mouth. But I do not have such an opportunity right now, so what do I do? Help me, give me the desire to please you, and I will be happy to open myself to receive.
The restriction is an unconditional action, and the blow occurs on the condition that I am ready to take pleasure if there is an adhesion between us. In the blow, my goal is connection. While in restriction there is no goal of connecting, only preventing yourself from receiving. I do not want any contact because I have no strength for more, only to restrict myself; otherwise, I will burn with shame.
The striking merger is already above shame. I no longer take shame into account, but only think about love and the connection between us, rising above selfishness. With a blow, I show that I am ready to receive a treat, not for myself, but only for the pleasure of the host.
At first we check our relationship with the host, and only when this spiritual connection of the heart has arisen can we translate it into action, and it descends into matter, into the desire to enjoy. Therefore, first of all, the decision is made in the Rosh of the Partzuf, in pure thoughts and feelings, disconnected from the clothing of the light in the desire, in matter. You can then perform this action, step by step, from easy to complex: the light of Nefesh, Ruach, Neshama, Haya, and Yechida.
From the 1st part of the Daily Kabbalah Lesson 6/17/19, Writings of Baal HaSulam, “Preface to the Wisdom of Kabbalah”