A defensive specialist in volleyball is just what the term implies. It is a player who comes off the bench and almost always replaces a frontline player when that player rotates to the back row. However, the rules of volleyball also allow a substitute called a libero, and the difference between the two roles confuses even some veteran volleyball observers.
A defensive specialist, or DS, is allowed to substitute for any player and take a position in either the front row or back row. However, in theory, a DS enters a game to play a back-row position. The DS usually rotates through all three back-row positions before she is replaced by a frontline player. A team is only allowed 12 substitutions. Every time a DS comes into the game, it counts as one of the 12 substitutions. When the frontline player replaces the DS, it counts as one of the 12 allowable substitutions as well.
A DS makes her mark by digging out drives and smashes, setting up teammates and returning serves. Digging is characterized by hustle and anticipation, including the ability to read the blocking and attacking angles spread out in front of the DS. While the frontline players receive most of the accolades, defensive specialists need lightening-quick reflexes and a willingness to dive and dig for anything they can reach. A defensive specialist is accustomed to floor burns.
A libero is easy to spot -- she wears a different color jersey from her teammates. Unlike the DS, a libero is only allowed to play a back-row position. She can substitute for any player who is in a back-row position and it is not counted as one of the team's allowable 12 substitutions. When a libero comes out of the game, she must sit out at least one play before she can return to the court. Liberos also are allowed to serve for one designated player.
Liberos and defensive specialists are allowed to be on the court at the same time. So they could replace two offensive players who have rotated to the back row. Using a defensive specialist effectively is a tricky business for a coach, since substituting a DS in and out of the game uses up two of his 12 allotted substitutions. Coaches use a DS in a similar fashion to the way a baseball manager uses a pinch hitter. But the libero can be shuttled in and out of the game at will and usually gets more playing time than a DS.