[kdf5]

The important thing to remember about force is to make sure to determine whether or not the ORIGINAL force is spent. If A fumbles and the ball would go in the EZ even though B touches or muff it, then you can't say there's a new force on the ball and that the original force still remains. So, in this video it's clear that A's fumble put it in the EZ. Now go back and think about B muffing it. It's clear that a muff by B wouldn't be a new force in this video since the fumble was sufficient to put it in the EZ.

Here's the rule on force, 2-13-1 through 2-13-4:

ART. 1 . . . Force is the result of energy exerted by a player which provides
movement of the ball. The term force is used only in connection with the goal line and in only one direction, i.e., from the field of play into the end one. Initial force results from a carry, fumble, kick, pass or snap. After a fumble, kick or backward pass has been grounded, a new force may result from a bat, an illegal kick or a muff.

ART. 2 . . . Responsibility for forcing the ball from the field of play across a
goal line is attributed to the player who carries, snaps, passes, fumbles or kicks the ball, unless a new force is applied to either a backward pass, kick or fumble that has been grounded.

ART. 3 . . . The muffing or batting of a pass, kick or fumble in flight is not
considered a new force.

ART. 4 . . . Force is not a factor:
a. On kicks going into R’s end zone, since these kicks are always a touchback
regardless of who supplied the force.
b. When a backward pass or fumble is declared dead in the end zone of the
opponent of the player who passed or fumbled, with no player possession.