From Wikipedia:

"The 12th man is a term commonly used to describe the fans within a stadium during American football or association football games. Since most football leagues allow a maximum of eleven players per team on the playing field at a time, the term denotes the attempt of a team's fans to help their team. The term has been claimed to be created by Texas A&M University in 1922. (see Origin and Texas A&M tradition below).

Due to the very nature of fans, their mere presence can have profound impacts on how the teams perform. Namely, the home team fans would like to see their team win the game. Thus these fans will often create loud sounds or chant in hopes of distracting, demoralizing and confusing the opposing team while they have possession of the ball; or to persuade a referee to make a favorable decision. Noises are made by shouting, whistling, stomping and various other techniques."

The Seahawks-Cowboys game is at Seattle on Saturday at 5pm PST.

Go Seahawks!