Question: What is the middle line? Is it the ability to act correctly between the two lines (left and right)?
Answer: The middle line is a marriage of the Light and desire. It is adhesion comprised of desire and intention, combined with the Source of Light, the Creator. By using the will to receive with an intention to bestow pleasure to the Creator, man uncovers the source of pleasure, the Creator. It turns out that desire and pleasure were merely a medium to find the connection between man and the Creator. These three components: desire, pleasure, and intention come together as one: I and the Creator.
From the 1st part of the Daily Kabbalah Lesson 9/22/10, Shlavei Hasulam, Article 17, 1986