The "Shell" drill has become one of the most popular defensive drills among coaches to teach and refine defensive fundamentals. Most coaches use the drill with four offensive players and four defensive players in a half-court setting and cover such aspects as ball pressure, jumping to the ball, help and recover, etc. Once you have covered the fundamentals, here are a few suggestions that will make the shell drill even better.
1. Start the drill at full or 3/4 court
I think it is more realistic to begin defensive drill work from the pickup level rather than the backcourt. So, if you play half-court defense, start the drill with the ball one to two steps above the half-court line and begin play. This gives your players an opportunity to practice defending cutters to the ball and denying.
2. Start the drill from the sideline or under your basket
How often do you practice against out-of-bounds plays? Here is the opportunity to prepare for certain actions (example: screen-the-screener) used by teams you will face. Choose the sets and movement your opponents like and incorporate them into the drill.
3. Run different entries
Most of the time, a coach puts two players up top and a player on each wing, hands one of them the ball and begins the drill. I don't believe you are forming game habits with this method.Start the drill with a down-screen or a back-screen or have the passer cut to the basket every time,or receive a back\screen to simulate a motion-type offense.
Build reaction and habits by mixing different actions and entries into the drill. For example, your next opponent has a certain set play you must defend. Take it and put it into your shell drill and work on defending it.You must decide what is important to you defensively and utilize the shell drill each day to constantly reinforce the techniques and habits of good defense.