[Source] A gravity lock was a type of portable, hovering tractor beam that could prevent a starship from taking off by anchoring it in place. That type of gravitational technology was developed by the Galactic Empire several years before the Battle of Yavin. Once clamped down onto a vessel's landing gear, a gravity lock could not be removed by force, and was nearly impossible to destroy. The only way to remove such devices was to dismantle them, which required knowledge of their inner workings. Some gravity locks were assembled by inmates of Imperial prisons.