From a sagittal view: This drill is to improve contact with the ball. It is important to first master how to contact the ball on a serve before learning anything else. Proper contact on the ball will allow the ball to go over the net and different serves will come from different contacts on the ball. For example: contacting the ball directly in the middle with force will allow the ball to float in the air. Another example is contacting the ball at its peak and flicking (flexing) the wrist with force will make the ball have a top spin. Also by mastering this drill it is easier to place the ball anywhere on the other teams side.
In order to do this drill a volleyball ball and a sturdy wall is needed. Throw the ball up and hit the ball towards the wall with palm of hand. For beginners start off a few feet from the wall then back up once contact gets better.
From an anatomical reference position the dominate arm is abducted to about 90 degrees, elbow flexed about 90 degrees by your head, and knees are flexed about 45 degrees. The other hand is throwing the ball in a flexion movement. Since this drill is to improve ball contact, quantitative movement not qualitative movement is important in this drill.