The other end of the wire is a socket that fits in the power amp. The black bit fits into a socket on the back of the subwoofer. It doesn’t power the woofer; power is provided by its own separate power supply.
Here we have the residue of my plumber’s clumsiness. He managed to get his foot caught up in the cable from my power amp to the subwoofer.
As you can see I have 3 wires and 4 possible holes. The permutations don’t particularly scare me, but blowing up either my amp or subwoofer (or both) does...