This is a top-quality LED taillight that can be plugged into the 6v controller light output of all Bafang and TSDZ2 mid-drive motors sold by Electrify Bike Co. (TSDZ2 requires an extra wire harness found here). The lights can be turned on and off via the display controls. When you put the display into night mode (hold down the plus button on displays like the 850C), the lights come on.
This taillight also has an internal voltage step-down converter, making it possible to also be plugged directly into an ebike battery up to 52v. This works on all ebikes but must be wired manually.
Unless purchased with a power tap, this light comes with bare wires and must be wired to the controller or battery. If connected directly to the battery, the taillight will be on whenever the battery is installed or switched on.