Check out the the Ultimate Resource for Man to Man Defense...
http://www.btball.com/mandefense

This is a video clip from our Man to Man Defense 4-DVD set with Jim Huber. In the DVD set, Coach Huber shows you how to build the defense step by step from the ground up. No guesswork! For more details, click on the link at the top of this description.

Here is why I believe every coach should use the shell drill...

1. Great for teaching and getting players on board.

The shell drill is great for teaching and allows you to demonstrate to the players exactly what your team defense should look like and to build confidence. It allows you to communicate the players how you want things to be done and why you want things to be done a certain way.

Are you closing out properly? Are you sprinting to areas? Are you in the right position? Are you guarding the ball properly? Are you properly rotating on help?

It also gets your players on board so they understand the big picture.

2. Great progression for developing your defense.

Most great coaches like to start in a closed, controlled environment to teach and build confidence before introducing an open, reactive environment. This would be a prime example of progressing from a shell drill to live scrimmaging.

When you are working on developing a player's shooting form, you're not going to teach them the form, then instantly progress them to a 5 on 5 setting and say, "Okay. Work on your shooting form." You're going to build the proper habits in a controlled environment. You're going to practice repetition after repetition to develop the skills and habits needed to be a good shooter.

Even though teaching defense is not the exact same as teaching shooting, you should still use similar philosophy when teaching your defense. The shell drill does that by building the proper habits and engraining proper defensive positioning through repetition.

3. Used to improve weaknesses.

Sometimes, during the middle of the season, you may find a few weak areas in your defense. The shell drill is a tool that you can use to go back to those areas, emphasize them, and clean it up to improve your defense.

4. Used to be more efficient with your time.

As a coach, you are always trying to be more efficient with your time. What's a better way to be more efficient with your practice time than getting 8 to 10 players on the court at the same time working on defensive and offensive fundamentals? This is especially useful if you don't have a bunch of quality assistants to run breakdown drills.
Your players also get to work on multiple aspects of defense all at once: on-ball defense, help positioning, close outs, defending cutters, defending screens, defending post play, rotations, etc.