Rich is right. You will get different answers. Here's what I do..,If the offensive player has such a head of steam that he's going to run defense over regardless then I will call charge. The plays that would have been nothing if he stays up I will have nothing or if it causes people to trip I will call a blocking foul.
There are things in rules I question based on how game is played and coached. As a coach and player, if I or my kid doesn't stay in for contact, falls early, then I don't expect a charge call. I don't know a single coach who will say he's entitled to fall early. That's something a rule maker came up with. That's my opinion. As the coach of the defense I'll sure take it but I don't expect it. I'm not sure who comes up with some of these rules.
And moving backward is different than falling early. Moving backward entire body goes back.