Pro Face-Off is a type of multiplayer mode, where both players play the same song, on the same difficulty, using the exact same notes. When playing Pro Face-Off, a meter sits in the center of the screen, between the two note highways. Initially, a needle sits at the center of the meter, and as the song progresses, the needle leans towards the player with the current highest score.

Pro Face-Off made its first appearance in Guitar Hero II, and can only be unlocked by completing Career Mode on Medium or harder. On later games, you don't have to unlock it.

A video of a Pro Face-Off: http://www.youtube.com/watch?v=JzX4Id4eO5E