Vintage Odd Ball Magic Trick

Two cords are threaded through a ball.
The ball is placed into a plastic case, with the cords extending out of the box through holes on the sides.
The spectator holds the box, and the cords that are running through the ball are also tied around the box and around the spectator's finger.
At your command, the cords pass right through the ball AND the box AND the spectator's own finger!