If I am throwing into a backwards fall correctly, I will have locked my partners spine and cause their feet to suddenly thrust forward (as if stepping upon a roller skate). Either one foot or both feet. The fall that results will not be a back roll.
The only time that back rolls seem to work in technique is when uke chooses to go before the terminal point in their posture where their spine is still not locked and they can still take a step forwards or backwards.
I can see that we are totally on the same page. If you are able to move backwards when falling, one of 2 things have happened.
1) Nage is letting you (i.e. they are not really throwing you)
2) Nage is has screwed up the timing and technique, etc. and is relying on their own strength.
Neither is desirable.