This is a second of three tests that was recognized by NHSTA as an indicator of having greater than a .10 BAC in one's system. The test is divided into two stages: (1) the instruction stage and (2) the walking stage. During the instruction stage, the officer is scoring you to see if you break the stance that you were instructed to assume (with one foot in front of the other) and start the test too soon (before finishing the instructions and being told to begin).
During the walking stage, the officer is looking to see if you step off a straight line, take the incorrect number of steps, fail to touch heel to toe (more than 1/2" apart), fail to count the steps out loud, incorrectly turn, or raise your arms more than 6" from your body. If you show two or more of these clues, there is a 68% chance that your BAC is above .10.