If any perturbation along will be amplified in time. If any perturbation along will decrease in time. Finally, if any perturbation along will keep the same distance. See Tabor for pictures. The matrix is now two dimensional and has two eigenvalues.
All orbits lead to the fixed point.
The trajectories curve and the direction with the eigenvalue largest in magnitude will approach zero fastest. All orbits lead away from the fixed point. The trajectories curve and the direction with the eigenvalue largest in magnitude will leave zero fastest. Stable in one direction, unstable in another. All trajectories lead to the fixed point and they spiral inward.