This is the "horizontal gaze nystagmus" test, a relatively recent development in DWI investigation. The officer attempts to estimate the angle at which the eye begins to jerk ("nystagmus" is medical jargon for eye jerking); if this occurs sooner than 45 degrees, it theoretically indicates an excessive blood-alcohol concentration. The smoothness of the eye's tracking the penlight (or finger or pencil) is also a factor, as is the jerking when the eye is as far to the side as it can go.

This field sobriety test has proven to be subject to a number of different problems, not the least of which is the non-medically trained officer's ability to recognize nystagmus and estimate the angle of onset. Because of this, and the fact that the test is not accepted by the medical community, it is not admissible as evidence in many states; it continues, however, to be widely used by law enforcement.