Okay, in the overall scheme of the game, this play didn't make any difference. Green Bay still won. I'm not a huge GB fan, but having grown up in Houston, I am predisposed to hate the Steelers!

With 37 seconds left in the Third, Rodgers throws to Swain, who (to me) catches the ball, starts to turn up-field, and has the ball stripped and fumbles. The play is ruled an incomplete pass, Challenged, and the call stands.
My question is, how was this NOT a completed pass resulting in a fumble? What then constitutes a complete pass?