I was at that game, and after the 2nd wash TD, and then the kickoff that almost went for a TD, the crowd was pretty much silent. for about 20 seconds it seemed like most people were set to give up, but as soon as Seattle took the field on defense to force the missed fg the crowd was more crazy then it had been all game.
Momentum is only important due to confidence it gives one team and takes away from the other team. A confident team will beat a team that is reeling and confused almost all the time. Which is why great teams are not affected as much by a change in momentum, because they are still just as confident as when they were up.