Combat models, albeit very rarely, may have their lines cut or otherwise broken by the other airplane, and can fly outside the circle. Most combat competition today requires that the airplane be equipped with a device that shuts off fuel to the engine if the lines are cut. This can work by either using the centripetal acceleration to keep the shutoff from pinching the line, or by using the line tension to hold the pinch-off device open. This way, the engine is not running and tends to decelerate very quickly instead of accelerating quickly as it would otherwise (since the drag of the lines is removed). This technique has proved to be very effective in practice. Many combat events also require the engine to be attached to the bellcrank with a cable, so that a mid-air collision will not result in the engine flying off and into the spectators. Competitors in some forms of combat competition are also required to wear helmets.