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

If the officer is unable to make this distinction from a slight nystagmus, the benefit of the doubt must be given the suspect. A common error is failing to bring the eyes out to side as far as they can go or returning the stimulus too quickly. This leads to an incorrect scoring. The criteria of no white showing in the corner of the eye must be met. This is also a common oversight of the officer. Nystagmus is commonly and incorrectly scored when the officer does not hold the stimulus out to the side for the required 4 seconds.  Fatigue Nystagmus may be evident when the eyes are held too long at maximum deviation, such as more than 30 seconds. 

In approximately 50% of the suspects, a physiological nystagmus will be apparent when the eye initially arrives at maximum deviation. This is due to a person not normally following an object all the way to the side with their eyes. Instead a person commonly turns their head to view that object. Consequently, a certain degree of uncomfortableness is experienced. This would cause a slight twitching of the eyes at this location, having nothing to do with alcohol consumption. If the officer brings the eyes out to the side and then immediately brings them back, he most probably scored this slight twitching inappropriately as a clue.