This is a safety device used on several kinds of recreational vehicles, (snowmobiles and Jet Skis mostly). It works like this. The operator of the vehicle will wear a wrist band with a cord that is plugged into a special switch. If the operator of the vehicle falls off, the tether will come out of the switch. This causes the vehicles engine to die, so it doesn't go roaring off unmanned where it could be a threat to people's safety.
I recommend this device for all recreational vehicles that you could possibly fall from. They are simply a win-win situation. They keep your snowmobile from being damaged, by making it stop when no one is driving. They keep other people from being hit by your runaway watercraft. Finally, they keep you from being dragged by your own vehicle in the event that you fall, and are caught up in the vehicle somehow.