Answer: “The brook that descends from the mountain” is a quality of Bina that comes down from Mount Sinai. Egoism (the dust of the golden calf) must be mixed with the quality of bestowal in order to produce a connection of Bina and Malchut to reach the second restriction (Tzimtzum Bet) that is a special symbiosis of qualities of reception and bestowal when they get combined and become ready for gradual and consistent correction during the so-called forty years.
The people, representing the quality of Malchut, stand at the very foot of the mountain, they don’t even have a drop of spiritual qualities, no strength to climb the mountain.
This brook that is flowing down (Bina) begins to mix with Malchut. This is great mercy of the Creator when He gives His force in order to mix with the egoism of a person and to begin raising him. And a person is required only to close his mouth, heart, and eyes and to want just one thing: to be included in this brook by at least one point.
From KabTV’s “Secrets of the Eternal Book” 5/11/16