It is my belief that that the Boost cut out "lever/catch" actually prevents the the throttle from being pushed past the gate. So if you want +12lbs be it for take off or in flight you still need to operate the Boost Cut out catch, this uncovers the gate and allows further throttle movement forward. With the catch in place the the Gate limits the throttle movement so max boost is the rated value (+9Lbs for the Spit MKII and +6.25Lbs for the MKI).
The division of Take off use and in flight use in the engine limitations section of the notes does not imo amount to much other than stipulating the desired 1 min take off limitation.
In other words Take Off Boost and Emergency boost are two different sets of limitations but the mechanism is the same. .... i.e. activate boost cutout to enable +12Lbs to become the max limited Boost.
Jpg below is from Spit MKI pilots notes dated June 1940
As can be seen this indicates removal of the Boost Cutout catch allows further forward throttle movement to achieve up to +12Lbs boost.
looking in the MK I manual to the same fuel system schematic as Banks illustrated above shows no gate per see in the MKI quadrant. As does the Throttle drawing in the MKI notes:
So a change in quadrant design seems to have occurred between the MKI and MKII. This commensurate with the increase in rated boost to +9Lbs in the MKII (as against 6.25in the MKI). However I don't believe this makes any real difference how you get +12Lbs boost in either case Boost Cut out switch needs to rotated forward.
I surmise that the inclusion of the gate in the MKII simply provides the pilot an additional tactile feedback for Rated (+9lbs boost in the Spit II) throttle position. So that if he has the the Boost cutout catch rotated forward then the Gate provides a tactile stop to +9lbs rated position.
In combat the pilot might enter the fight with Boost Cutout switch already rotated forward ready to go, so he doesnt have think about finding and pushing the cutout switch forward in the heat of combat. Then he can easily get +12 by simply rocking the throttle past the gate and pushing it all the way forward. If not in dire straits then he knows pushing the throttle to the gate will give him rated +9lbs boost.
Getting back to limitations these are not Instant failure values but rather wear and tear values on the engine.