Siyyum, the ritual done when one finishes learning a tractate of Talmud, emphasizes that every such ending is not a real end but only a time out.
The siyyum is a full stop at the end of a sentence, but not the end of the utterance.
When the study ends, it also begins.
Not only can one — as the text recited at the siyyum says — begin and finish studying other books or tractates, the siyyum is no more than a temporary break in the process of studying that very same tractate itself.
In fact, the most important words in the siyyum text are “hadran alakh” which literally mean, “we shall come back to you” — namely, a promise to study this tractate again.
Therefore, we are called upon to repeat the study of the same tractate over and over again.
It is not that we are asked to learn it by heart, for even if we know very well every word and letter, we are still asked to study it again.
Some of the greatest Jewish scholars, among them the Vilna Gaon (Rabbi Eliyahu son of Shlomo Zalman Kremer of Vilna, 1720-1797) and the Rogatchover (Rabbi Yosef Rozin of Dvinsk, 1958-1936), had eidetic memory; they retained each and every word and sentence they ever studied — and yet they never ceased to study.
These scholars went over the same books time and time again, because every new cycle of study reveals new aspects of the Torah, facets that can be revealed only in the second, or 100th, time.
This is why whoever finishes studying a tractate promises to repeat it again and again, regardless of whether he knows it forward and backward or whether he does not really remember it all that well.
However, in the siyyum text we do not only say hadran alakh (“we shall return to you”), but also hadrakh alan, “you [the tractate or book] shall return to us.”
This means that if one does not do the real work, if one does not learn it again and yet again, then all the material he has learned will come back to him.
Namely, he will inevitably find himself engaged in a multitude of problems and questions relating to the very book that he had closed and put away.
Thus, every finishing point of whatever part of the Torah is only a “recess.” After a certain point, one must once again start studying everything from the very beginning.
–Rabbi Adin Steinsaltz
From an essay, "We Shall Come Back to You," by Rabbi Adin Steinsaltz