How do anodes work on small outboards/sterndrives?

I watched an episode of “Modern marvels“ about corrosion. It explained that they are sacrificial pieces of metal with electricity flowing thru them( below the waterline on ships). How do the anodes work on say an outboard motor or a sterndrive? They only bolted on with no electricity to them. Can you please explain how the water knows to corrode the anode first?