Originally Posted by
rholland1951
As I headed up High Street, an awkward shift resulted in my chain folding back and catching a link in the front derailleur cage.
What in some circles is referred to as chain suck. With just the right combination of tooth spacing, chain ring separation, chain flexion, and stretching the chain gets stuck on a tooth on both chain rings. If you do it with enough force on the pedals it can get suck pretty hard.
The unfortunate consequences include dragging the chain against the chainstay and if you were pedaling hard enough pulling the RD forward hard enough to break it. That happened to me on a commute one rainy night about 1/10th of the way home. In the blink fo an eye the rear wheel locked and spun out and I hit the ground hard. Fortunately only the RD and my headlight mounted on the left side of the handlebar came away hurt too badly to be fixed. I had to rebuild the wheel but that was easy.