No, not another criteria but a different criteria. Ball goes backwards directly out of a quarterbacks hand and it is a live ball. Pretty basic, really.
Honestly, I thought that was the rule anyway.
You have most definitely changed the definition of what is an incomplete pass verses a fumble by adding another criteria (whether you call it another one or a different one is just semantics). Now, in addition to the referee having to determine whether a QB's arm was moving forward or not, he has to determine if the ball lands behind the QB or not. That's an additional requirement.
The current rule on a ball knocked loose from a QB in the act of passing is:
1. QB arm moving backward equals fumble.
2. QB arm moving forward equals incomplete pass.
It doesn't matter which way the ball goes so that's not a criteria that needs to be considered.
What you want the rule to read is:
1. QB arm moving backward equals fumble.
2. QB arm moving forward and ball goes forward equals incomplete pass.
3. QB arm moving forward and ball goes backward equals fumble.
You've added ball movement as a criteria that was never there before.