Years ago when I first started coaching I attended and worked a camp at the University of Arizona. One of the main daily drills Coach Olsen and his staff worked on was the shell drill and defensive rotations. Coach Olsen explained to me that this was was one of the most important aspects of their practice and that most players need constant work in this area.
Over the past 20 years of coaching I have learned how true coach Olsen's comments were regarding the shell drill and defensive rotations. It might seem simple to have your pistols up and head on a swivel trying to locate you man and the ball, but this has proven to be one of the most difficult aspects of the game for players to master. Through daily repetition, players start to form both individual and team habits that will allow them to position themselves more correctly in live situations. Since there are a variety of offenses the players have to use the habits learned through the shell drill to position themselves correctly no matter what offense is being run. The team success increases as the the individuals work together defensively. The shell drill is one of the best drills a coach can use to teach both individual and team responsibilities.
This link is from a FIBA clinic. It is a set of drills that takes the regular shell drills and makes them competitive situations teaching the same principles. I hope you enjoy .