A defensive specialist in volleyball is just what the term implies. He is a player who enters and almost always replaces a front-line player when the player turns the last row. However, the volleyball rules also allow for a substitute called libero, and the difference between the two roles can confuse even some veteran volleyball observers.
A defensive specialist, or DS (for its acronym in English), is allowed to replace any player and have a position in either the front row or the back row. However, in theory, a DS enters a game to play a back row position. The DS usually rotates through the three positions of the last row before being replaced by a front-line player. A team only allows 12 substitutions. Each time a DS enters the game, it is counted as one of the 12 substitutions. When the first line player replaces the DS, it is also counted as one of the 12 allowed substitutions.
A DS makes its mark by looking for successful shots and throws, ordering teammates and returning services. The search is characterized by constant movement and anticipation, including the ability to read the blocking and attack angles distributed in front of the DS. While front-line players receive the most accolades, defensive specialists need quick reflexes and will find whatever they can achieve. A defensive specialist is used to scratches on the floor.
A libero is easy to find, since he wears a shirt of different color than his teammates. Unlike the DS, a libero is only allowed to play in the defender position. You can substitute any player who is in a back row position and is not counted as one of the 12 team substitutions allowed. When a libero leaves the game, he has to sit down at least one game before he can return to the court. Liberos can also serve for a designated player.
Liberos and defensive specialists are allowed to be on the court at the same time so they can replace two offensive players who have turned the last row. The effective use of a defensive specialist is a difficult business for a coach, since the substitution of a DS inside and outside the game uses up to two of its 12 assigned substitutions. Coaches use a DS in a manner similar to the way a baseball manager uses a pop-up hitter. However, the libero can be changed and exit the game at will and, in general, has more playing time than a DS.