Bat and helmet.
If this drill is done as a group drill, make sure all players have plenty of space and are wearing helmets. For younger players you may want to not use a bat as a group drill and then have them use a bat as part of a batting practice station.
Each player will get into his stance
The coach will simulate a windup and deliver an imaginary pitch. The coach will yell inside or outside as he delivers the pitch.
The player will coil, stride, and then swing at an imaginary pitch.
Things to watch for:
Have the player repeat the drill, stepping out of the box and getting the sign from the third base coach each time.