This transformation leaves UNCHEANGABLE the vectors (x,y) in the plane with x=y that lie on the line y=x. Hence, these vectors (x,x) are eigenvectors with the eigenvalue of 1. This transformation CHANGEs the vectors (x,y) in the plane with x=-y that lie on the line y=-x, orthogonal to the line x=y. The transformation transforms each such a vector into the opposite one. Hence, these vectors (x,-x) are eigenvectors with the eigenvalue of -1.