Horizontal Gaze Nystagmus is the involuntary jerking of eye as it tracks from one side to the other.  Law enforcement administers a test under NHTSA guidelines to determine if a Maryland driver displays this involuntary jerking of the eye as the cop passes a pen or finger in front of the driver's face.  The driver is told to hold his head still and follow the item with his eyes only.  If the driver displays the nystagmus or involuntary jerking, the officer takes that as a positive "clue" in attempting to determine sobriety.   The problem with the nystagmus test is that there are a large variety of other reasons that can result in a person displaying nystagmus.  See the law section on nystagmus in this website for more information also see the Schultz case and the Horn case for more.