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

In particular, NHTSA does not support the allegation that onset angle of Nystagmus can reliably be used to estimate BAC, and considers any such estimation to be misuse of the Horizontal Gaze Nystagmus test. The NJSP requires that the student obtain 20 practice tests prior to being considered certified in the Horizontal Gaze Nystagmus. There is no time limit on when these tests can be obtained. A refusal can not be considered a practice test as the evaluation of the suspect must be corroborated by a blood alcohol reading.

The stimulus must be placed twelve to fifteen inches in front of the suspect's face. The stimulus should be held above eye level, so that the eyes are wide open. Due to narrowness of certain individuals eyes it becomes more difficult to make a fair evaluation of the nystagmus unless the eyes are wide open. 

The officer cannot simply believe he sees nystagmus, it must be obvious and distinct. The officer is trained to have the suspect look into a quiet background, away from the police cruiser overhead lights and traffic passing in close proximity. This may induce a condition known as opto-kinetic nystagmus. Numerous visual or other distractions may impede this test such as rain, snow, drizzle and traffic, to name a few.