From all of the pics, it looks like it functions exactly like what is on my AD 216s and 235s, except that instead relying on a bimetallic spring to automatically change the flapper from the "heat the intake" to "divert heat away from the intake" it is adjusted manually.
There is no restriction of the exhaust going on at all. All the flapper does in my AD trucks and in the OP's truck is direct hot air to the manifold, or direct hot air away from the manifold according to what the engine requires to operate smoothly.
When the engine is not up to operating temperature, the flapper needs to direct hot air toward the intake to heat up the intake runner so that raw fuel does not fall out of suspension from the air/fuel mixture.
When the engine approaches full operating temperature, the flapper needs to direct hot air away from the intake manifold to prevent fuel/air mixture from overheating causing poor performance.
What is the middle position for? That is for operating in extreme cold conditions.
I am pretty sure that in January in northern Minnesota an AD engine with a bimetallic spring, the spring will hold the flapper in the middle, half open position.
The concept is very similar to choosing between having a manual choke or an automatic choke.
