The entrance guard is used two different ways. With the “teeth” down it acts like an entrance reducer and to keep mice from getting into the hive. The other side are “holes” that allow you to contain the bees within the hive and at the same time allow ventilation for the bees within the hive. You will want to contain the bees when it is dark outside so that the foraging bees are not kept outside of the hive.
The containment can be because you or your neighbor is having a pool party and you do not want the bees flying around the pool. This is an excellent easy way to contain the bees quickly within the hive.