2 players stand on either side of the net. One player overhand passes to their team mate who sets the ball back for a spike down the line.
On the other side of the net, one blocker stands at the net and attempts to cover a cross court shot, while a digger is at the back of the court, and has to attempt to dig the ball either to their team mate or back over the net.
Two players are involved in defense action. They need to work together and cover at least half of their side of the court.
If coach wants to focus on digging mostly, then the blocker can become another receiver at the back of the court, ready to receive and dig a cross court shot.
New hitters rarely miss because of their arm. They miss because of their feet. Build a reliable approach, then time it with your setter.
When your hitter gets stuffed, five teammates standing and watching is how you lose the point. Good attack coverage turns a blocked ball into a second swing.
Coaches spend hours correcting arms in serve receive when the real fault is almost always the feet. Fix the movement and the platform fixes itself.