What happens when y=0 in case of scaling a plane?

Scaling the plane along positive y-direction has no effect, but scaling along negative y makes the plane face downwards. What happens when y=0 in such case of a plane?

Scaling not only changes the distance between vertices, but also the direction of their normals. When the Y scale is 0, the computer does not understand which direction to direct the normals, and therefore equates them to zero.