Black Diamond ATC Belay Device

About five years ago, I bought this with the hope of one day doing a rappelling descent while stream fishing and reaching places that are inaccessible to everyone. However, I've never used it since then... I haven't even threaded the rope through it.
I no longer know how to use it or what its purpose is, so I'm offering it to someone who can make use of it. Thank you. (Comes with a bonus item)

Compatible rope diameter: 7.7mm - 11mm
Weight: 60g