Partners stand facing each other about arms distance apart, one of the partners is the tagger who tries to touch the partner on the knee the other has to avoid being tagged whilst remaining close.
The players should bend mainly at the knees not at the waist (danger of banging heads if bend at waist).
Both players should be in a well balanced ready position so they can move and change direction at any time.
You can play this game by one being the tagger and changing when they get tagged or both being taggers at the same time in a competition, counting how many times you can tag your partner.
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.