With the addition of Wii MotionPlus you will see the characters mimic your hand movements in real time. For example, the game will know if youre doing a backhand or a forehand swing, how high or low your tennis racket is, or if you are going to perform a topspin or backspin just based on your movements!
Grand Slam Tennis also gives you an option of playing with or without the Nunchuk. If you play with the Nunchuk, the analog stick will be used to move the characters manually. If you decide against using the Nunchuk, your character will move automatically towards the ball like in Wii Sports Tennis.