The NFL rule is poorly written and should be changed to better align with the Rugby rule.
When analyzing the motion of objects using the laws of Physics, one must first choose a frame of reference.
The intent of the Rugby rule establishes, in essence, the frame of reference being relative to the position of the two players at the time of the release of the ball... and aligns with the first part of the NFL rule.
The problem arises because the second part of the NFL rule uses a different frame of reference, namely it switches from the Rugby version (relative to the position of the players at release) to the position of the ball when released and when caught relative to the position on the field.
A lot people here have used the words "forward momentum" when explaining their take on what happened. Technically, they should be using the terms "relative and absolute velocity". Yes, the ball has momentum but momentum doesn't explain the Physics involved.
Since Russ is running with the ball, he and the ball together have a relative velocity in the direction of the opponent's goal line (we'll establish that direction to be positive (+) in our frame of reference... the speed is not in question... just the direction of the velocity). And when the ball leaves Russ's hand, the velocity of the ball relative to the players is negative (-), that is, away from the opponent's goal line. That is why the play looks legit.
But... when determining the absolute velocity of the ball, both components of the ball in motion must be mathematically taken into account... the one towards the opponent's goal line (Russ running with the ball at the time of the release) and the one away from it (the flip backs towards Davis who is running behind and to the side of Russ).
The velocity of the ball while in Russ's hand going toward the opponent's goal line is slightly greater than the velocity of the ball tossed backwards towards Davis... that difference in velocity, taking into account how many seconds the ball was in flight, is why the ball travels forward approximately one yard after leaving Russ's hand.
Hope that makes sense.
Signed, retired Boeing engineer with a BS in Mechanical Engineering.