The main reason for a higher ready position at the net than at the baseline is to keep you from having to lift the racquet much to meet high volleys. Lifting the racquet is slower than dropping it, so you're better off having a long drop to a low ball than a long lift to a high one. You'll also usually slice a low ball, and a racquet that's already dropping will lend itself to slice. The left hand on the throat of the racquet will help bring the racquet back.
For discussion of the grip and other essentials common to all backhand volleys, see Basic Backhand Volley: Photo Lesson.
