This Magneto type kill switch contains circuitry which is normally open (not complete) when the lanyard clip is in place on the switch.
When lanyard is pulled and the clip is removed from the spring-loaded switch, the circuit is allowed to close. This completes a circuit to "ground" (such as a grounding block, the battery, or a suitable area on the engine itself) — short circuiting the magneto's output and shutting the engine down.
WARNING: Always test switch for proper function before each use. Replace lanyard if worn, cut or frayed.
Available in bulk or retail packaging.
Why have a kill switch?
Many open cockpit powerboats as well as personal watercraft have a safety "kill switch" installed, which shuts down the boat’s engine if the operator falls or is thrown overboard. The kill switch prevents the boat’s ignition system from working unless a special "key" is inserted into a spring-loaded mechanism on the switch.
The key is specific to the kill switch - which may be a manufacturer's proprietary switch, or an aftermarket switch like these from Sea-Dog.
The "key" is attached to one end of an extendable lanyard, and the boat’s operator attaches the other end to their body - their belt, or wrapping it around their leg - such that it doesn’t interfere with steering, etc.
If the operator is thrown overboard, the lanyard will pull the key out of the kill switch, shutting down the vessel's engine. This prevents the boat from becoming a run-away vessel that could impose a danger to other vessels or swimmers, and also allows the operator to swim back to the vessel stopped and re-board it.
Note — Always keep a spare lanyard with key onboard and readily at hand - so if several people are on the boat, and the operator is thrown overboard (taking their lanyard and key with them), one of the remaining passengers can use the spare kill switch key to operate the boat in case the overboard pilot can’t make it back to the boat their own.