We can compare the ascents and descents to a pump in which a piston goes up and down. When the piston goes down, air is expelled from the opening in the wall of the pump. But then this valve closes and the piston begins to rise and creates a vacuum, thus pulling out part of the material from the cylinder, and it goes into your vessel. And you have to build a Masach (screen) on top of it.
From the 1st part of the Daily Kabbalah Lesson 1/24/13, Writings of Rabash