What is the purpose of the emergency engine kill-switch lanyard ('Hombre al Agua' / Cortacorriente) on an outboard motor or PWC?

1. To lock the propeller when mooring at a marina
2. To regulate fuel flow for optimal fuel economy
3. To instantly shut off the engine ignition if the skipper falls overboard or is displaced from the helm station, preventing the runaway boat from circling and striking people
4. To sound an automatic horn signal during fog