Two players (as a team) stand about halfway between the service line and the net on one side, and two other players on the other side halfway inside the service court as well.
They practice volleys hitting back at each other as follows:
– one team hits only cross-court and the other team only down-the-line.
Switch after about 5 minutes: the one who hit cross-court will hit down-the-line this time and so on.

Note: Very good for volley control, placement, and it can be a great workout. Winners are not allowed, players should try to keep the ball in play.