Using "Paint Holes" brush in Terrain

Greetings,

I cannot seem to find a good explination of the "paint holes" option in Terrain. It doesn't seem to do anything when i use it.

I would like to hide/ remove a bit of terrain in the way of a doorway to an underground building.

I found a video or two that said to put a cube though it and hide the cube, but how do i hide the visuals of the terrain as well ?