Some where in one of the Threads is a proper description of what the Valve is there for.

Generally what I got from it is that it is to prevent a situation that can happen if you are heavily loaded down or towing something up Hill.
That would cause your Cylinder Pressure (I am speaking of the Pressure in the Cylinder when the Fuel is burning) to rise and at the same time increase the Boost Pressure that you do not need at that particular time because increased Boost would increase the Cylinder Pressure even more.

I also thought that the Valve did something to the Transmission?

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84 300D, 82 Volvo 244Gl Diesel