To tell you the truth I'm not sure what happens. I have turned engines backwards numerous times for various reasons. I can tell you that turning it backwards would be sort of like trying to pedal a ten speed backwards. The tention should be between drive gear and driven gear if you go backwards you are driving through the tentioning device (derailer on the bike). if the chain is slack enough you might pull the tentioner back loosening the chain and jump a gear. Also its said that dragging a chain backwards over a rail can cut it. I have bever seen it.
Bosch Master, ASE Master, L1
33 years MB technician