Shell defense is where you have four defensive players and four offensive players. 4 on 4 on 4 is a defensive/offensive drill to help on backside defense.
You have have three teams of four. one team starts up the floor and the defense gets into shell 'D'. And the offense must pass the ball three times, after the three times you are live four on four. If the offense gets the rebound the defense must run a line drill, and the offense goes to the other end of the of the court and does the same.
It will help your team get prepared for backside 'D'.
You may also play a zone type of defense, or you can tell each group to play different kinds of defense like one three one, one two two, and man.
This basketball drill is licensed under a Creative Commons License. This drill is from Pål Degerstrøm at www.degerstrom.com/basketball.