In this test, the officer passes a pen in front of your face and you are asked to follow it with your eyes. If your eyes exhibit a jerkiness (nystagmus) during the passes, it indicates intoxication. However, the officer will have to admit that nystagmus can, and does, occur naturally in many people, and that stress and bright lights (which are present at most traffic stops) can bring on nystagmus.
Walk and Turn
The police officer will ask you to keep your hands at your side, walk heel-to-toe 9 steps (counting each aloud), turn and walk back. They will be looking to see if you're off balance, if you stop, if your heel and toe didn't touch, if you used your arms for balance, and more. There are many challenges to this test: the slope of the road, presence of gravel, heavy clothing in the winter, high heels, passing traffic dangers, distracting squad lights or spotlights.
One Leg Stand
Only pelicans are physically qualified to do this test - the rest of us bipedal creatures have a tough time! They will ask you to put your feet together and then raise one foot and count to 30 using "one thousand one," etc. The same test conditions that are troubling for the Walk and Turn apply here.