There are conflicting reports I have read on listservers of no problems and critical problems in rolling bikes backwards with Duopar rear derailleurs and chain deraillment to a freewheel cog other than the cog under which the upper pulley is situated. Perhaps it is an adjustment but then those several who have reported problems didn't respond back to the listserver 'oh, you were right, an adjustment fixed it.' Given that there are much better derailleurs available, one can twiddle it as one wishes. Mine is in a box and I've moved on to better and sturdier performance. As usual, your mileage may vary.