That black box is the external shifting box for the Speedhub. The hub has no return spring, and uses dual cables in a "pull-pull" configuration from the twist shifter to the hub. A thumb screw holds the box to the mount, and allows easy disconnect in the event the rear wheel needs to be removed from the frame.

It's a little intimidating to rig up, especially for a first-timer, but once it's set, you'll not touch it for a long, long time.