The second standard field sobriety test is the horizontal gaze nystagmus test (HGN). During the HGN test a driver is asked to follow an object, usually the officer’s finger, back and forth across the person’s field of vision. Research has shown that drunk drivers exhibit a telltale wobble in their eyes when following objects, which is what the officers are watching for. The wobble is reflexive and not within the control of the driver so there are no tips or advice to avoiding it.
Finally, the heel-to-toe walk is another standard component of a field sobriety test and involves the suspected drunk driver walking back and forth in a straight line. In addition to the walk, officers often also throw in additional requests, such as a certain number of steps or seconds, something they use to check your attention to detail. The officer performing the test will watch for a lack of balance, swaying or failure to perform the test as requested as indications of impairment.
Source: “Police Identify Drunk Drivers through Signs of Impairment,” by, published at OhioBar.org.