While riding home from the LBS, I saw a bike whiz through a red light. I only caught a quick glimpse lasting a few seconds and couldn't figure out how the guy was turning the crank while the chain remained stationary. The bike had derailleurs from which the chain was sagging as with chain suck. I've been trying to solve the riddle. Maybe the chain had just dropped and the cranks were still turning? He was definitely accelerating and the cadence was even, not sudden overspin as when a chain drops. Maybe it was a left drive and the chain was hung from the derailleurs as an ornament? I never got a good look at the crankset to see if the chain was on the ring [that couldn't be] or hanging on the bb shell. I was too busy ogling the sagging chain from the rear derailleur as he was hammering and wondering what in the hell did I just see and how was that possible. Maybe the setup is his sense of humor for knocking road bikes, i.e. derailleurs are dead. Anyone who doesn't ride or build/service their own bikes would likely have ever noticed something was amiss. Why didn't I chase him down you ask? I just had right carpal tunnel release surgery last week and am riding essentially one handed. Yes, I flipped the wheel and am using a front brake to slow me down as I "coast" up to the stop lights. Post-op wasn't a good time to learn how to stop with only one hand on the bar. Best of luck with the riddle. Many thanks for helping me to solve it. Live well, ride your bike!

He was probably cross-chained (small chainring - small cog) or, the bottom section of his rear deraileur could have seized causing the deraileur to sort of lock in a forward reaching position. I've had that sort of thing happen mountain biking before.

why bother?

happens to me occasionally: chain drops off the crank, but isn't caught between the stays/frame. a quick up or down shift pops the chain back onto the crank.

many thanks for the replies...the critical info was that the chain afixed to the rd and cassette was never moving, but the bike accelerated...no need for any replies...would shatter the mystery for me like when finding out there was no Santa...put some fun between your legs, ride your fixie!