When you hold down the middle mouse button and drag, a selection box forms in a parallelogram (or oval?). This will keep the same outlined shape and copy the terrain inside.
When you left-click somewhere else it will imprint the same thing (sort of as a brush). This would be useful as a quick eraser, a way to make patterns, and would generally make creating maps easier.
When you click the middle mouse button again, it goes back to the regular menu brush you had.