These are used to increase ventilation in the hive and reduce congestion. The added ventilation means less bees allocated to cooling the hive and more bees foraging and an increased honey yield. In cool weather it helps keep the brood chamber warmer so the so the queen can lay right to the bottom of the frame.
In warmer weather, when your hive populations are at their largest, it provides a cluster space in the hive which can reduce bearding.
The reduced congestion and increased ventilation also helps reduce swarming.
A Slatted Rack is installed, between the bottom board and the bottom brood chamber.
Can also be used with a Screened Bottom Board.