Belive it or not I actually picked it up from you! ( -> link).
I did some testing and found the trick to it, it deletes everything after the last occurrence of the specified character. In your post you used the character "." which windows doesn't seem to allow file names to end in, so it to gets trimmed off, leaving the rest of the file name.
It actually seems to have an (inverted) parallel to the "Enter char to delete up to:" function from hitting f4 on the command line. (by the way does anyone know how to harness the fx functions within a script without user input?)
So to answer you questions I am unaware if it was intended or is documented.