This description of the Horizontal Gaze Nystagmus test posted by California DUI Lawyer.

Everyone has physiological nystagmus. It is not visible to the naked eye. It becomes more obvious and distinct as the blood alcohol level increases. The normal physiological purpose of this nystagmus is to exercise the eye muscles, lubricate them, and prevent atrophy. Remember, the jerkiness must be distinct and obvious to be scored a point. 

The final clue is known as Angle of Onset . This is the most difficult to evaluate. If a suspect exhibits this clue in one eye, the probability increases that all the other clues will be seen as well. This does not work in reverse though. If a suspect has any of the first two clues, it is no guarantee that the third clue will be there. Because the 45-degree angle is a key factor in assessing a suspect's degree of alcohol influence, it is important to know how to estimate that angle.