Copy on change

I was hoping it would be easy (well, maybe it is and I just didn't figure it out) to copy a file whenever a change is saved to it. There are a few ways to do it. I tried creating a rule that applies whenever "date modified is greater than date opened", it seems those values are the same for the most part so that didn't work. I then thought I could do something like "datemodified greater than 5 minutes ago" or something. Even "datemodified = today" would be nice. I guess the problem with those rules would be that they would keep applying until the condition fails, which could be 5 minutes for example.
The best option would be "on modification". That would be awesome.
Any help is appreciated.
Thanks,
-jason
The best option would be "on modification". That would be awesome.
Any help is appreciated.
Thanks,
-jason