The Null Zone is a window of DS RPM in which the unit can be set, to reduce a lesser amount of power, or make no correction at all.
The Null Zone is useful for fine tuning the amount of power reduced at different points on the race track. While you may need to cut 100 HP in the early part of a run, that same amount of reduction may be too much in the later part of the run.
By using the Null Zone to tune the system, the user can keep the amount of retard optimized to where the vehicle is on the track.
Another example of how the Null Zone can be implemented is on a clutch car like a Top Fuel Dragster. At a DS RPM of say 6000 RPM, As the clutch locks up and the drive shaft and engine begin to couple one to one, the tires will often start to slip. Reducing power at this point will cause the clutch to fully lock prematurely and actually cause tire spin.
Using the Null Zone to limit the amount of power reduced, or prevent reduction completely between 5700-6200 RPM can avoid this situation altogether.