Baal HaSulam, The Study of the Ten Sefirot, Part 1, “Table of Questions and Answers for the Meaning of the Words”: Item 15. What is Zakh. Behina Aleph (the first phase) of the will to receive is defined as more bright (Zakh) than the three Behinot (phases) that follow her.
“Fine” and “rough” are defined relative to the will to receive. In the root phase, Keter, there is no depth (Aviut) to the desire to enjoy at all, but only the will to bestow is given, instilled by the Creator in every creature. Therefore, the first phase (Aleph) is the beginning of the creature, its desire to enjoy, and it is brighter and finer than the other three phases that follow.
This is because it hasn’t yet experienced many impressions from the influence of the Creator, the Light, which makes it the brightest. The Light hasn’t yet developed it to the depth of desire suitable for use. It is still the very beginning, the embryo of the desire, and in this sense, it’s the closest to the Light.
It doesn’t yet have any independent action and its own reaction. However, when it begins to react at the end of the first phase, it causes the emergence of the second phase, Bet.
From the 3rd part of the Daily Kabbalah Lesson 6/20/2011, Talmud Eser Sefirot