Equation
Lieutenant General
They might have spied on Broncos' games. Belichick has been caught video recording their opponent's coaches conversations before. It's hard to believe that the Seahawks would have been able to figure out Manning's signals at game time. It's equally hard to believe that they would have waited until game time to figure it out. They have done their homework. Well, what can I say? Manning was outplayed.
Seahawks defensive coaches and players had watched films from previous games to see how Manning signals were used against other teams, that's how they can figure out the Broncos habits in every situation and conditions. It's legal for both teams to obtain game films from previous opponents before the game, BUT it is illegal to film and watch the other teams play calls and signals during game time, although you can take notes and distribute it at half time.