(2) No light accomplishes its purpose until it emerges from and then returns to its Source. In other words, the light descends with great power all the way down to the bottom. Afterwards, as it ascends, it leaves down in its place below the level in question, while the light itself goes up. The given level then remains there, built in its place within the structure, and so in the case of all of them.
The proposition has two parts: Part 1: The Sefirot shine... This is a prefatory statement setting forth a fundamental aspect of the Sefirot. Part 2: After the descent... This explains the above statement.