Slatted racks provide dead air space below the brood chamber. This layer of air helps to keep the bees cooler in summer and warmer in winter. In the summer when populations are high, bees congregate in this area which reduces congestion in the hive, spreads out the heat load, and facilitates ventilation by fanning. This increase of space and lessening of heat seems to decrease swarming as well. In the winter, when the entrances are reduced, the air space within the slatted rack acts as an insulating layer between the brood chamber and the cold area below the hive. It also removes the brood nest further from the drafty entrance.
The slats line up with the bottom of each frame in the brood super.