A pass (forward) has to be thrown from behind the line of scrimmage, but doesn't have to cross the line of scrimmage. If dropped, the play is blown dead, and the clock stops. A lateral (backward) can be thrown anywhere on the field as many times as you want. If dropped, the ball is live. You can throw an overhand, underhand, two hand, any hand pass across the entire width of the field as long as the receiving point of the ball is behind the starting point.