Quote Originally Posted by Headwrapper View Post
I have friends that are super into biking, but I've never been able to get into it. Prefer hiking or like... moving heavy things.

But if I were to guess that's either the tension control thingy (tensioner, I think?) or the thing that moves the chain when you shift gears. Leaning towards the first one.
The correct answer is, "Geisha internals"