1) In pairs the players have one ball betwen them. 2) Standing on opposite sides of the court player 1 throws the ball as if spiking/serving to their partner who catches it. 3) Player 1 then has to lay down on their back as quickly as possible and get bback onto their feet. 4) Player 2 then repeats steps 2 and 3. 5) The players then repeat the steps for a limited time.
The important thing is to get your players to step forward with their left foot before throwing. If the player is right-handed get them to use their hips and torso to rotate during the throwing action.
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.