'Hitting zone' is the distance over which your strings are facing/pointing towards your target during the hitting phase of the stroke (the forward swing pre, on and post contact). A longer hitting zone increases chances of a consistent contact point (timing) and increases potential for more racket velocity (speed over a given direction). Use of the non-dominant hand during the one-handed backhand key in this.
See here 10 backhands in a row against the wall aiming for the cross. Pretty proud of shots 8 and 9! (This was 'take 1' I promise!)