How can it have an overall effect when its action is targeted at such a small area (the glabella)?

There is a particular anatomical advantage in that all the muscles in the central and upper part of the face converge in the glabella area.

In this way, a small electrode placed on this spot inhibits all these muscles.

The muscles at the back of the head become dominant, drawing the skin of the forehead and ears backwards and opening up the face.