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