At trial the prosecutor and arresting officer will do their little "dog and pony show" about how the driver's jerky eyes indicates that he must be intoxicated. The officer will testify (usually) that he found all six clues when administering the test and that the only logical conclusion is that the driver was intoxicated at the time he was driving.
Of course if you take out a stop watch while viewing the video it becomes evidence quite quickly that the officer didn't administer the test correctly. Either he moved the pen too quickly or too slowly in front of the driver's eyes or he held the pen too high or too low when making the passes.
Do you file a pretrial motion to suppress the test, file a motion in limine requesting the opportunity to take the officer on voir dire before he testifies about the HGN test, do you conduct a vigorous cross examination of the officer on his mistakes or do you gloss over the test and move on to other topics?
Here's a little hint...in most jurisdictions when the jury watches the video all they see is your client standing still with the officer waving a pen in front of your client's face. The jurors don't see your client's eyes jerking. In fact, you can often score points with the jury by asking the officer if your client stood still, didn't sway and didn't move his head back and forth.
Jurors are far more concerned with how your client did on the Walk and Turn and One Leg Stand tests because that's when they get to see if your client "looks drunk." Those are the tests to attack hard and often.
I tend to ask a few questions about the "pen and eye test" and then move on to more fertile ground -- unless the officer butchered the test to such a degree that it casts his entire investigation into question.