There's two things I think you might be talking about. The first (and I think least likely) is the bottom board, the floor of the hive. On your common bottom board, the hive entrance is taller when one side of the bottom board is up than it is when the other side is up. I vaguely, distantly remember way back in some old book that this is on purpose, that the bottom board is intentionally made to be "reversible". But for the life of me, I've never seen a single article or heard a single talk discussing this use of this feature of the bottom board, or when the correct time for "reversing" it might be. I've always had em deepest-side-up and that's how they stayed.
The other thing I think you might be talking about is the entrance reducer. It's the wooden cleat that sits across the hive entrance, blocking most of it except for a small notch. At the appropriate time, you rotate this cleat so that a larger notch (larger entrance) is made; and then finally you remove the cleat altogether so that the whole entrance is open.
There are nuances involved; but generally, you want to leave the smallest entrance there until they've drawn about 7 or 8 frames in that first deep fully - in other words, you switch to the slightly larger entrance size at the same time as you put on your second deep. Once that second deep is 70% drawn, remove the entrance reducer entirely.