The Zohar, Chapter “Tetzaveh” (Command), Item 18: And when the light of Butzina illuminated in the letter Yod, from the part of illumination of the Yod, the three points of the Yod—Holam, Shuruk, Hirik—expanded, became two lines, and the middle line connected to them in Hirik. …Then these four points expanded and became one Heichal…. hence there are only four points in each of the two letters Hey, and not all nine, as in the Yod. This is because each has only one Behina of the Yod—one has Toch and the other Sof.
Baal HaSulam explains that the tip of the letter “Yod” which was made by the Creator “out of nothing” includes everything within it. The Light and the point “made out of nothing” constitute complete opposites; the Light is round and so is the darkness which stands opposite to it.
How does the Light, by acting within desire, generate various forms of desire? If the Light and desire stand against each other, why doesn’t the Light influence desire evenly? The secret is that the Light influences desire not as good influences evil or the Light darkness. That being the case, a small black point “Yesh Mi Ain” (existence from absence) would simply expand. Rather, the Light carries the “Thought of Creation” to please the creature; it bears the Creator’s desire to not only fulfill the creature, but to also transfer the Creator’s inner essence to it, thus making the creature similar to the Light.
In other words, the Light carries within itself two qualities that impact the “dark point.” This is why the dark point not only expands, but also develops in different directions. This gives rise to the forms which we call “letters.”
From the Evening Zohar Lesson 5/31/10