The following images shows the different mode shapes of the wing.
In mode 6, the upper surface of the wing displaces downward and the lower surface of the wing displaces upward. Since the amplitude is arbitrary, the two surfaces may even cross each other. Obviously, this is impossible, but there is nothing in our model that says this cannot happen. It is important to realize our model only applies to small vibrations and the displacement amplitude does not represent a real-world value. When the upper surface and lower surface displace in this fashion, the mode is called a "breathing mode."