Baal HaSulam, The Study of the Ten Sefirot, Part 1, “Table of Questions and Answers for Topics”: Question 102: What are Sefirot de Igulim? When there is no differentiation of above and below among the four Behinot (phases) in the Ratzon (desire), they are regarded as four Igulim (spheres, circles), one within the other, like onion skins.
In the very beginning, in the Light of Infinity, there were no differences between above and below, due to the absolute authority of the upper force which ruled over the desire to receive completely equally. But having made the restriction and the screen and having gone through the expansion of the worlds from Above downward and the return of the souls from below upward, the desire arrives at the state when it has multiple properties, differences, and peculiarities, all of which become enclosed in an integral system. And hence, no difference between them remains.
This means that in the very beginning, in the world of Infinity in State 1, desire was “round” (Igulim), as it is in the very end, in State 3. But in the beginning, desire didn’t have distinctions since they were invisible and everything looked the same.
In the very end, at the final correction (Gmar Tikkun), there are no differences either. Having revealed in this desire enormous number of properties, varieties, and details, we bind them all together into one integral system where every element is as vital as the entire system. And hence, there are no differences between them!
This is another type of “circles” (Igulim) which contain great number of “straight” ties (Yosher). But these straight connections reach a state when they lock into a circle.
From the 3rd part of the Daily Kabbalah Lesson 7/25/2011, Talmud Eser Serifot