A special teams position used in either the punting formation or kneel situations where the player in this position is lined up behind the offensive line and act as defense for the punter in punt situation and the quarterback in kneel situations. The upback is considered one of the "special teams" and are part of a team's backup or alternate players.
During punt situations, this player is one of the blocking backs that are positioned 1 to 3 yards behind the offensive line. The key objective of an upback during a kick formation is to block offensive players.
During a kneel formation, the upback will focus on trying to recover the ball in the event of a fumble during the snap. For example during a kneel formation the quarterback will have two upbacks directly behind him in the event he fumbles the ball so as to obstruct the other team from recovering the ball.
An upback is allowed to act as a receiver for direct snaps as well. This allows for a fake punt with the upback receiving the snap instead of the punter.