I understand the physics behind a ball floating forward because of momentum, but forward passes are judged on whether the ball leaves the hands backwards or not. That's not always easy to tell.
Surely an easier way to judge if a pass is forward is whether the receiver was behind the passer. Just use the same methodology when judging whether a player is offside from, say, a kick. The Bunker seems to be able to check quickly if a player is behind the kicker, why not behind (or in line) with the passer?