Air assault is the movement of ground-based military forces by vertical take-off and landing (VTOL) aircraft—such as the helicopter—to seize and hold key terrain which has not been fully secured, and to directly engage enemy forces behind enemy lines.[1][2] In addition to regular infantry training, air-assault units usually receive training in rappelling, fast-rope techniques and air transportation, and their equipment is sometimes designed or field-modified to allow better transportation within aircraft.