This is an excerpt from Volleyball Skills & Drills by American Volleyball Coaches Association (AVCA).
Purpose: To improve players' ball control by practicing a drill that should be very easy, but which proves to be annoyingly difficult to complete—hence the name
Setup: It is difficult to run this drill with fewer than 10 players; 12 to 14 players works best.
Execution: Players position in lines of two as passers (P), setters (S), and hitters (H) on both sides of the court. A ball being tossed from the first passer on one side to the setter on that side starts play. After tossing the ball, the passer joins the setter's line. The setter sets the ball to the hitter and moves to that line. The hitter two-hand passes the ball over the net to the passers on the other side of the court and goes around the net to join that line. The passer forearm passes the ball to the setter on his or her side and continues in that fashion until an error is made, at which point the entire process begins again. The goal for this phase is for the ball to cross the net 10 times consecutively. When that has been accomplished, the drill is started over with both passers simultaneously tossing a ball to the setter on that side, and the drill continues as before. Now, however, with two balls in play, focusing becomes much more difficult. Movements need to be faster, and communication becomes important. Again, the goal is for the ball to cross the net 10 times consecutively. If enough players are available, introduce a third ball as the drill progresses (figure 10.2)
Coaching Points: In addition to ball control, this drill is very useful in helping players focus on their responsibilities at all times and not just the ball. This drill also helps with reading on defense and in fostering communication. This would be an excellent warm-up drill except that it often takes a significant amount of time to complete.
Variations: When players have mastered this phase, hitters are instructed to use a tip or controlled spike instead of the two-hand pass over the net. During this phase, passers are forced to begin to read the hitter's approach and where they'll most likely hit the ball.
Read more from Volleyball Skills & Drills by the AVCA