Adjust Artillery

[morvael]

From help:

Adjust Fire button Use to adjust the impact hex of an already plotted. Artillery observers will have less delay added for an adjustment than the Battle Group HQ (if it is an HQ class unit, ability lost if changed to e.g. a tank), then more for other units.

and again:

Keep the barrage up continuously, just adjust the guns forward 2 or 3 hexes per shift (select the gun on the barrage dialogue, and hit the HE icon, it targets its last adjustment at low delay so there is not a long wait. Then use the Adjust Fire button to shift 2 or 3 hexes if required. This is how to do a 'creeping barrage' in SP without the lengthy delays. Think of it as a preplanned fire program).

For me it seems I never see the Adjust Fire button active. I think this is a very important feature to be able to shift fire a bit without full delay required for new bombardment.

Is it possible as the manual says, and what are the prerequisites to be able to adjust fire?