As you will see in this video clip, he's on his back before the goal line, pushes himself with one leg, then lifts the ball over his head past the goal line. You'll see a several second delay before the side judge rushes in to signal the TD. That several seconds allowed Johnson to scoot on his back and lift the ball past the goal line.
http://www.youtube.com/watch?v=vLfR0o_zhIg&t=34s