Earlier today, I saw this bike with an absolutely bizarre drive-train. It looks like a single speed, but it has a derailleur on attached to it. The derailleur does not move horizontally, but only along a vertical axis. The bike is a Mongoose Alta, but I could tell that it was heavily modded. The bike is not originally a single speed, and has a different derailleur.
Does anyone know why this would be a desirable modification? I'm really curious.