What does the black wire do on a 5-wire full wave regulator?

Has anyone used one of these 5-wire full wave regulator rectifiers before? I was told to use this for my 110cc Honda Wave full wave conversion.



I understand all the wires except for the black one. What's the point of having the black wire connected to the ignition switch circuit?

The ignition switch circuit shuts off the CDI which shuts off the bike. Why would you need the regulator-rectifier to *also* be on the same circuit?