Get players into pairs and have them standing across from each other on the court, and with a ball between them. The player with the ball jumps and baseball passes the ball to their team mate, throwing it into the floor so that it bounces high.
The exercise is similar to spiking, plus when jumping with no approach, the leg muscles are forced to work even harder.
If the task is too difficult for some players, which is entirely possible at first as they get used to us, you can ask them to swing their hand back prior to the jump.
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.