There are two players: one of them (A) serves, and the returner (B) sends the ball back with a slice (groundstroke) short inside the opposite service court.
The server comes up to play the short ball; play from there on.
In a real match situation the server, depending on his skills, can back up to the baseline or move up to the net.
Note:
The goal of this practice drill is for the returner to learn how to get a baseline player out of his comfort zone who would have to take quick decisions of whether move up to the net or back away from it.