The restriction on passing is that if you have moved the ball past your own line of scrimmage, you cannot throw the ball forward relative to the field, only sideways or backwards. It is not often tactically advantageous to do this, although it does happen on occasion[1]. Since getting 10 yards in 4 downs suffices to retain possession, it's not worth the risk of turnover unless there's 4 seconds left on the clock and you're down by one point (as in [1]).<p>[1] <a href="http://en.wikipedia.org/wiki/The_play" rel="nofollow">http://en.wikipedia.org/wiki/The_play</a>