A great man to man defense is built from the ground up, and that means breaking down certain actions like being in the gap, help, communicating, etc. In order to focus on these details you need to start out with defensive breakdowns, and teach your team how to handle each type of situation. That is where the shell drill comes into play. The shell drill is going to be 4 on 4 in the half court, and it is going to allow your team to practice specific situations that you will encounter when playing man to man half court defense.
This specific shell drill is going to work on where to be in the help/gap when the ball is 1 or 2 passes away, and then also on how to handle offensive cutters. Basketball drills like this are really important for your team in learning to play defense, but they are also great for working on man to man offense as well, because they allow your players to get reps on offense too.