Yom Tov is a concept of teshuva, as is known. How is this known? - because we all go through the same motions. The ways of teshuva are as such: when a person first starts to repent he is uplifted and brought close to Hashem. And, then afterward he is seemingly distanced from Hashem, and the person has a tremendous downfall. He falls to distant places. When this happens he needs to strengthen himself with all of his might, and should not allow himself to fall again, Heaven forbid. If he is able to keep himself from falling, his first downfall becomes the catalyst for an ascent. He merits to uplift holy sparks, which are an aspect of lost items veiled in lowliness. Through teshuva he is able to return and ascend to holiness even more than before.
This is an aspect of chol hamo'ed, which is right after the closeness and holiness of the first days of the festival. This time is synonymous with a person's descent from holiness when a person must be cautious. Even though a he is now in a state of "coming forth," because he fell and descended from his holiness to a state of chol, nevertheless he must sanctify himself with the holiness of mo'ed, which is a concept of the prohibition of doing labor on chol hamo'ed. However, labor is permitted to be done on chol hamo'ed only if one will suffer a financial loss. A person must know and strengthen himself, for doing such labor is not a complete descent from the sanctity of the mo'ed, Heaven forbid, for it is also a time of mo'ed and Yom Tov. In such a case, a person is prohibited from falling from his place except to prevent a loss.
Thus, when a person sanctifies his descent with the holiness of chol hamo'ed, he then returns and uplifts himself to the holiness of Yom Tov more than before. This corresponds to "a descent is the catalyst for the ascent." And this relates to the last days of Yom Tov, which radiate even more light than the first days of the festival, This is a concept of the splitting of the sea on the seventh day of Passover and the great feats the Jews saw at that time, as Chazal says in the Mechilta, "What the maidservant saw at the sea was more than Yechezkel ben Buzi." (Shemos 15:2)