As you've surmised, you can't use a fixed sprocket with a derailleur. It's not just a matter of trying to stop pedaling.
Even trying to simply slow down (which basically puts back-pressure on the chain) will damage the derailleur and/or the hanger portion of the frame.
Perhaps, if you were extremely deft with the rear brake, you could get away with it. But the first time you tried to slow down simply by pedaling less rapidly, you would risk the damage mentioned above.
Fred Rednor - Arlington, Virginia (USA)
> This is quite likely a very ignorant question, but
\r?\n> I'll ask it anyways and risk providing a few chuckles
\r?\n> at my expense.
\r?\n> I recently acquired a bicycle with a flip-flop hub.
\r?\n> It is currently set up with a freewheel on one side,
\r?\n> and nothing on the other. I'd like to take advantage
\r?\n> of the other side, but it seems that it would not be
\r?\n> practical to set it up with a fixed gear. I suppose I
\r?\n> could do it, and use the derailleur essentially as a
\r?\n> chain tensioner, but it seems that if I were to stop
\r?\n> pedaling (out of habit from many years of exclusively
\r?\n> riding freewheels), I could potentially damage the
\r?\n> Am I correct in presuming that the only suitable
\r?\n> arrangements would be to use fixed gear on both sides,
\r?\n> or freewheels on both sides?
\r?\n> Thanks in advance,
\r?\n> John Barry
\r?\n> Mechanicsburg, PA, USA