I don't understand why anyone would want to backdash *cancel*...
To backdash normally, you have to make sure that there's already some space between you and the opponent at the moment before you tap 4. Space where BBs won't quite hit you, or will barely do.
And of course, you do that when you feel that the opponent is going to attack.
I feel that everything I said is pretty obvious, though, so maybe you were asking for something else...
Edit: Just like "GIs on reaction", you can backstep popular strings on reaction. Although I can't think of many, but there are two from Siegfried as an example.
66KA, backstep the A, punish hard.
66B,WRAA, backstep the first A, interrupt second A by punishing ultra hard.