The shield polarity was an attribute of a deflector shield.
In 2371, when the USS Voyager came under bombardment by organic elements inside a nucleogenic cloud being, Captain Kathryn Janeway proposed reversing the shield polarity, to no effect. (VOY: "The Cloud")
During its passage through the space of the Swarm species in 2373, Voyager encountered a single Swarm vessel that fired a polaron burst into its shields. The burst changed the shield polarity to a rotation of 92 gigahertz, alerting the much larger Swarm fleet to Voyager's presence. (VOY: "The Swarm")
While in command of the Kraylor medical transport "Nightingale" in 2377, Harry Kim reversed the shield polarity at a crucial moment above the Kraylor homeworld, when they were caught in the tractor beam of an Annari warship. This broke the tractor beam lock and gave them enough momentum to reach the safety of the Kraylor defense perimeter. (VOY: "Nightingale")