This is an offensive drill because you are working to score runs. But this is one of the baseball communication drills that work best because everything is done on the field with an offense and a defense. Moreover, the coaches can be in the "box" directing the runners.

Purpose: To create an environment in which the offense must execute a given offensive play.

Procedure: Group hitting and executing and a full team defense playing the ball.  

Coaching Point: Third base coach will give an offense signal, the offensive players will acknowledge they received it, and then they must execute the sign. As the offense is working on their execution, the defense must execute their actions to stop a run from scoring or getting an out.

Note:  I usually did this with the starting line-up and base coaches.  We would run this drill exactly like a game and the team had to execute.  Every now and then call in a pinch-hitter, pinch-runner, or courtesy-runner.  Make it as game-like as possible.

All of this is done with batting practice pitching. For this drill we usually went 7 minutes with the starting 9, 7 for the reserves, and then 7 more for the starters.