The notch is actually in the top board which is beneath the hive cover.
The hive cover is a little larger than the hive boxes which allows for a bee-sized gap between it and the top board. In a pinch, the bees can enter/exit from the top of the hive if necessary although I've never seen them do it in my hive.
To be clear, there's nothing under the rock except the metal wrapping over the hive cover. The rock's just there to help keep the cover in place when the wind blows.
