Works the same as a regular distributor. Difference is the tach drive mechanism is integral with the shaft. It turns a gear which spins a cable mechanically linked to the tach. The cable is similar to a speedo cable, it spins inside a housing.
Generally these are a 'race only' part since there is no vacuum advance provision. You need a mechanical tach like a Moroso/Jones type for it to work. S.W. also had one. Kind of a relic at this point but still cool.
I have basically NOS MP aluminum one for a small block. Got a vintage Moroso tach as well, planning on putting it in my Duster at some point. I don't have a vacuum advance on my car right now so it would just be a straight distributor switch with the tach drive.
Think early Pro Stock/Super Stockers from the early '70s.
