Several eye examinations are included in the field sobriety test and the drug recognition expert program, because they're very highly correlated with levels of blood alcohol and impairment.
The most commonly used test is the lateral gaze nystagmus, which is the involuntary jerking of your eyes as you move them to the extremities. It's very noticeable for certain types of drugs; you cannot control it. You have absolutely no voluntary control over those movements of your eyes. That's one of the tests that is used.
You can also do it vertically. Certain drugs will also show vertical nystagmus.
Some of the other eye tests include looking at the reaction to light, or simply looking at the pupil size. Some drugs dilate pupils; some drugs constrict them.
Those sorts of things are very important and critical to the DRE process.