What is Field Sobriety Testing?

Legal Definition
Field sobriety tests are a battery of tests used by police officers to determine if a person suspected of impaired driving is intoxicated with alcohol or drugs. The US National Highway Traffic Safety Administration (NHTSA) began research in 1975 on how to test suspects for impaired driving. The NHTSA developed a series of tests that police officers could use when evaluating suspected impaired drivers. By 1981, officers in the United States began using the organization's battery of standardized sobriety tests to help make decisions about whether to arrest suspected impaired drivers. The tests were designed to indicate intoxication associated with a blood alcohol concentration (BAC) of 0.10%.

After some US states began lowering their BAC limits to 0.08%, a study was done to see if the battery could be used to detect BACs at or above 0.08% and above and below 0.04%. This was done to deal with the changes in the laws that led to lower legal BAC limits across the US.
-- Wikipedia