The training exercise is shown with piston movement from the left side with 2 balls.
The exercise trains the players the correct moving rhythm and ability to change pace during passing.
When 1st ball reaches the opposite backcourt the 2nd ball is put into play.
In the exercise it can be difficult to obtain correct depth in the piston movement, but it is an effective piston movement and passing exercise, which also must be trained to the opposite side, where the balls moves in the other direction

If there are extra players these can be placed with 2 players on each rear player position.
They then perform the retracting to the same side as the passing and the second player is ready to move forward at once.