Obviously, the first priority for defensive players is getting the ball up in the air, but as Oregon State assistant coach Emily Hiza explains here, the key to really helping your team is controlling digs in a way that allows the setter to deliver a well-located ball to the attacker.
This drill can be done with any number of players. The goal is to dig a ball into a target area and catch it. The best dig is one that the player catches in the small box. The second best dig is caught in the bigger box. Third best is catching it anywhere on the court. And the least desirable dig is not catching it at all.
If the player who follows you in the drill makes a better dig than you do, you're out. The winner is the last player "standing."