On 4th down, you can't fumble the ball forward and have the ball recovered by anyone but the original ball carrier. Prevents intentional fumbling. Since another player recovered, it comes back to the spot of the fumble.
if you fumble forward on 4th down the running back must recover or the ball comes back to where fumbled. That was before first down line.
rule was instituted so that players dont fumble the ball forward on purpose on 4th down knowing they wont get past the first down marker famous play back in the 70s with dave casper fumbling for oakland and someone recovered the ball in the endzone to win the game in the waning moments while the rule is fine, for the replay officials should not have overturned it if it took that long to look at....that means they were never sure and the play should have stood as a first down
