Answer: Essentially, the right and left lines are diametrically opposite to one another. The right line is absolute bestowal, and it does not know nor does it want to know what reception is. While the left line is the receiving Kelim (vessels) which do not understand what it is to bestow.
But any one of our desires, impressions, or thoughts necessarily consists of a combination of Bina (the bestowing desires) and Malchut (the receiving desires). They can only be separated through the power of the Light, when I "suppress" one in relation to the other. Also, the right and the left lines both have their own three lines. We are incapable of separating a thing in its pure form.
The breaking soldered them together for eternity by the Light of the highest power. It fused them together to the depth of their core so that not a single detail or particle was left without containing both elements in it. For this reason, in reality, the right line is the bestowing Kelim which work on the receiving Kelim, and at the same time, in the left line, the bestowing Kelim hide beneath the receiving Kelim. Once I attain the middle line, I acquire the Kelim which have both these properties combined in them.
The interconnection that I attach to the middle line is the force of the Upper Light or, to be more precise, it is the force of the Creator, the Thought of Creation that has preordained and evoked the breaking. This way, by beginning to combine the two lines into one, I use the force of the Creator Himself.
I plan and intend to reveal Him specifically, and the middle line connects me to Him, the Source of the breaking. In the middle line I attain the reason behind the breaking and the Creator’s intention. By rising above nature, I merge with the One who exists above it. This way, the Creator conceals himself in the middle line, and I touch Him due to its uniting force. This is where the meeting point of the Creator and the creature is.
From the 1st part of the Daily Kabbalah Lesson 9/28/10, “What is Silver, Gold, Israel, the Nations in Work”