If volleyball players think they are starting the attack pass to soon, or the trainer thinks the attackers come in too soon, this is a good exercise.
The playmaker is positioned with a box of balls. He has to throw up the ball for himself 5 times or less. The attackers doe not know how often this happens, and can only leave when the set-up is done, not before.
The attackers will have to come in more aggressively, building up more speed and thus more height, if there is a good brake pass.
If this goes well, the trainer can decide to move the playmaker closer to the attackers, so the distance the set-upper has to cover is shortened. The attackers have to be even more alert.