Rev Counter sensing from alternator - how does this work?

[Refueler]

Seller confirms that Sensor which attaches to outside of alternator :

Diesel Engine RPM Tacho Sensor UNIVERSAL For Vehicle Truck Boat Yacht Waterpoof | eBay

and Meter :

52mm Car Marine Tachometer 0-4000 RPM Diesel Engine Tacho Gauge With Hour Meter | eBay



should work together ....

I've ordered both and will update later.