Most outboards have a lanyard attached to a kill switch, also known as the man overboard switch. When this lanyard is unplugged, the exciter is grounded stopping electricity from reaching the CDI unit (or the primary winding of the coil in an older outboard). This is how the kill switch stops the engine. As well as stopping the engine when you need to, removing the lanyard is a great way to stop the spark plugs from firing when you want to turn the engine over with them out as a part of your diagnostic process.