Bug 174 on 12lbs boost. Review please.

[JtD]

+12 lbs boost was only usable for take off and at low altitudes, and it fell of rapidly, in contrast to Spitfire I pilot notes which notes its effective up to rated altitude.

Now how would they technically do that, make the boost of 12" drop off quickly instead of having it follow the outside pressure? What did the automatic boost control cut out do on the Spitfire II if not disable automatic boost control?