Quote: bikerpie "Don't know where you got that from, as I see no mention of tackled or not on that page. It talks about the relative positions of the passer and receiver only.
'"
Because of the ambiguity in that definition.
Is it the relative positions of the players when the ball is released, in which case a player can pass the ball forward and the player 'behind on release' may run on to the forward pass. So clearly that cannot be it.
If it is the relative positions of the players on catching the pass, then a player passing a ball backwards who is tackled after release can end up behind the ball they passed when the second player catches. Thus the relative position on catching cannot be used properly (hence my classic example, first video on a well known video sharing website for "Forward Pass" @2.36).
If it is the relative positions of [iwhere the players would have been had each carried on running at the same speed etc[/i** then you may as well ignore the players and work with the velocity that the ball already has, and if it's forward velocity is reduced* by the passing motion. The relative positions of the players could easily be ignored in this interpretation, which is the one I have always understood it as. Hence the requirement to "pass backwards".
EDITS
* Or more accurately not increased.
** This is equivalent to saying the pass is only taken relative to the passer (catcher ignored) and that they have passed behind them (heading relatively backwards at the instant of release, ignoring any other factors after release).