An overwatching unit typically takes a vantage position (usually a high ground or tall structure with good defilade) where it can observe the terrain far ahead, especially likely enemy positions and movements. This allows it to act as a warning system against hostile aggression and provide effective covering fire for advancing friendly units.
A U.S. Marine Corps M1A1 Abrams tank assigned to Delta Company, 1st Tank Battalion, provides overwatch security in support of Operation Dynamic Partnership in Shurakay, Helmand province, Afghanistan, Feb. 12, 2013.