One quick question what is the purpose of the switching valve in the exhaust manifold? I can find references on how to check if it's working properly, but not its purpose.

 

Twin scroll. At low RPM/vacuum, it closes and directs exhaust gasses to hit the turbine at a sharper angle. This helps spool up. As RPM and boost builds, the actuator opens up and allows exhaust to hit the full tubine.

There are testing procedures in the factory and Haynes manuals.

 

That's part of it, the other important part being the much higher gas velocity as a result of the reduced flow area. Hitting the turbine blades faster spools it up quicker.