Answer: The guarantor creates a vessel that resembles the Light. When we are still connected with external connections, this is called guarantee. So through mutual guarantee we receive the Torah, meaning the Light that Reforms, the elixir of life, the Light of correction.
The Light of fulfillment doesn’t yet reach the first guarantee because the vessels are not yet ready for it. You left Egypt only now; Pharaoh still sits within you. You brought these vessels from Egypt and these vessels are terrible: They are all just hatred, envy, lust, honor, and dominance. Everything that is in you is comes from your ego.
And you have a point in the heart through which you somehow succeed in connecting with others, and we are ready to reach mutual guarantee. According to this pre-condition, we only receive the Torah, the Light that Reforms, and only after this do we make a Zivug de Hakaa (binding by striking) with the Light, clarify our vessel, and receive within it for the sake of bestowal.
But the guarantee is the vessel, and the extent of our connection in regard to the law and terms of guarantee determines the size of this vessel.
From the 4th part of the Daily Kabbalah Lesson 5/30/14, Writings of Baal HaSulam