The
umpire (U) stands behind the
defensive line and
linebackers, observing the blocks by the
offensive line and defenders trying to ward off those blocks — looking for holding or illegal blocks. Prior to the snap, he counts all offensive players.
During passing plays, he moves forward towards the
line of scrimmage as the play develops in order to (1) penalize any offensive linemen who move illegally downfield before the pass is thrown or (2) penalize the
quarterback for throwing the ball when beyond the original line of scrimmage. He also assists on ruling incomplete passes when the ball is thrown short.