The screen (Masach) is an anti-egoistic force that blocks the way of the Light. It allows a part of the Light to enter the spiritual Partzuf (the soul), in order to become the “Inner Light,” while the other part of the Light cannot be received for the sake of bestowal and remains outside as the “Surrounding Light.”
The Inner and Surrounding Lights are opposite to one another, even though it is the same Light. The screen splits them by relating to each of them differently. It is like two friends that come to my home and wish to come inside. I let one of them in gladly and I turn the other away harshly. How will the two former friends feel about each other now, after I’ve separated them!?
The same thing happens here. The Light comes as a single whole, but is split into two parts by the screen. These two parts differ from one another, but only with respect to the screen’s attitude to each one. That is why the two Lights strive to cancel the screen so as to unite once again; they feel that the screen is in their way. The Inner Light wants to cancel the screen so that its friend would be accepted as well. The Surrounding Light says: “If you don’t want both of us, then you’ll get neither!”
Then the screen is left without a choice and revokes its decision. It ascends and expels all the Light. So what is the decision it revokes? It is the decision to split the Light in two. The barrier that has been put between them must now be removed. This process is called the weakening (or thinning) of the screen. The unity of the Light is stronger than the barrier that the screen wishes to lay through it, and thus the Light defeats the screen with the power of its unity!