The police officer who administers the Horizontal Gaze Nystagmus test is taught to look for three clues in each eye. There are two eyes, so the HGN test has a total of three clues. This test is carries the least amount of weight with the jury, in my experience. First the officer is looking for lack of smooth pursuit. Secondly, the cop tries to determine whether there is nystagmus at maximum deviation. Third, the officer is looking to find the onset of nystagmus before 45 degrees. That makes a total of 6 possible clues.
There have been many a citizen accused where the officer found a total of 6 out of 6 clues on the HGN and the judge or jury found the defendant not guilty. However, if an officer believes that he has found at least four "clues" then he or she will treat the results as indicating probable intoxication. In other words, the decision point on the HGN is 4/6 clues.
Nystagmus is the involuntary jerking of the eyes. It occurs naturally in us all. It is important for the jury to hear the cop testify on cross examination that alcohol merely increases nystagmus, it does not cause it. There are dozens of causes of increased nystagmus, and if the police officer did not question the citizen about potential causes like diabetes or concussions, then that must be brought out even if the defendant does not have those conditions because the defendant does not have to testify! It must be explained to the fact finder that one reasonable doubt is what the jury needs to return a not guilty verdict.