To clarify, yes, the chain came off the large chain ring and was dangling around the crank arms. I'm not entirely sure what happened after that. I saw it hanging there and was in the process of slowing down so I could stop and put it on, and the next thing I knew I was ass-over-tea-cups. The only thing I can think of that would make the bike suddenly flip like it did was if the chain actually went under my back wheel. That could have snagged it tight like a wire snare. From the looks of my utterly destroyed derailleur, I wouldn't be surprised.