When executing swinging volley, you should contact the ball at least at wasit height with your forehand grip. With the swing volley, a player can put himself in an offensive position.
The swinging volley is an extremely aggressive shot. To help keep that aggressiveness from translating into wildness, most players use topspin. The position of Maria's racquet so far below the ball here indicates that she'll be brushing up the back of the ball heavily.
Maria Sharapova execute Swing Volley inside Service line
AO 2012 Sharapova and Azarenka Forehand Swing Volley Winner