Both players (A and B) rally down-the-line: A hits forehand groundstrokes, B backhand groundstrokes (assuming both are right-handed) – see graphic below.
After the ball travels 6 times over the net, player A will change direction and hit the ball cross-court, to which player B will chase and attempt a put-away shot down-the-line. Player A will not chase this last shot; the drill ends with B’s down-the-line to the open court “winner/finish”.
After about 5 minutes, players change roles: B will change the direction of the ball for A to practice down-the-line put-away winners.
This drill should also be done with players rallying on the other half of the court (player A with the backhand side, B with the forehand).