The police officer may ask you to stand on one leg while holding your other leg 6" off the ground for a period of 30 seconds while counting from 1 to 30 by 1,000s. The police are looking for four clues to see if you (1) put your foot down, (2) hop on the foot that you are standing on (to keep from putting your other foot down), (3) raise your arms more than 6" from your body, and (4) sway from side to side (i.e. unable to hold one's balance).
This is one of a battery of three "tests" that NHTSA has recognized as giving some indication of having more than a .10 BAC in your system. This particular test has been given a 65% reliability of predicting that the person's BAC is above .10 which is 15% greater than flipping a coin to determine whether or not someone has more than a .10 in his or her system.