The Entrance Reducer fits snugly in the opening between the bottom board and your first brood chamber to control the size of the entrance to the hive. When it's cold or your hive is small or vulnerable flip the reducer to a smaller opening to make it easier for the guard bees to defend and slow down traffic. Keep an eye on the hive when you notice bearding it will need to be removed so that it is easier for the bees to fan and bring down the temperature in the hive.