Over time, the midfield position has developed into a position of specialties. During play, teams may substitute players in and out freely, a practice known as "on the fly" substitution. The rules state that substitution must occur within the designated exchange area in front of the players' bench. Teams frequently rotate the midfielder specialists off and on the field depending on the ball possession. Some teams have a designated face-off midfielder, referred to as a "fogo" midfielder (an acronym for "face-off and get-off"), who takes the majority of face-offs and is quickly substituted after the face-off. Some teams also designate midfielders as "offensive midfielders" or "defensive midfielders" depending on their strengths and weaknesses.