I think he means the reversible bottom boards where one side gives a 3/8" opening and the other side a 3/4" opening. I picked up an abandoned hive that was on the 3/4" opening (never had seen one before), so I put a reducer on it for the winter and this spring i tossed it. When it landed I noticed the 3/8" opening side was up.
I love my migratory tops/bottoms which, btw are interchangable. If you knock off the front slat of the top cover it becomes a bottom board, add a slat it's a new top cover
Forgot to say, if that's what the original poster was talking about I would personally always leave it on the 3/8" opening, if you leave it bigger it's a pretty big opening to defend if the hive got robbed, also in the winter the mice will thank you. My recollection after speaking to another beekeeper about it was he flipped in the late spring to the larger entrance and then flipped again in the fall. If I were to ever use them I would always keep the small entrance and use a reducer when necessary, our temps can swing from -20 to 110 here.