That's what I thought as well. But after looking at the wiring diagram and everyone's comments (including some confirming that it does work) ... been thinking. Note that even on the + side the wire will alternate between current flow and no current flow as the points open and close, so there is a signal.

Our resident auto-electrician phunkdoctaspock says it works off induction, which is why it's a direct wire (only a loop of wire goes into the tacho).