I like to play where if a ball carrier falls over on the board, there is a fumble. I then put him upright and drop the ball from about 3 inches above his back. All unegaged players except the fumbler can pivot and run to recover the ball. Upon recovery, all unengaged players can pivot and the recovering player can run with the ball. I usually only have a fumble once every 6-10 games, but a fumble can be devastating when each team only gets two offensive posessions.