[hab_in_exile]

If the player releases the ball, backwards, while moving forward at 18mph, it is going backwards and will land (absent a wind) at a location behind where it was thrown. There is no exception to this, as the creation of momentum is impossible (in our universe, if we starting playing football in other universes all bets are off).
Earlier someone said that if it goes backwards at 5mph, the net is a forward of 13mph. Nope. If it is going backwards at 5mph, then it is going backwards at 5mph. You are mistaking the application of force with the velocity I think. The velocity (and direction thereof) is the only thing that matters, the relative application of force (a little forward, a little backward) does not matter, only the net result of velocity.
Did Bill Nye Science guy ever tackle this topic?