Sodium filled valves is nothing special. It has been used for ages (since the 60's) in the exhaust valves for cooling. I think first for aircooled aircraft engines because they tend to run hot.
It works on the basics that sodium has a very low melting point, so it turns to liquid and it cools the valve from the inside by transporting the heat to the valve stem faster than it would through the steel. They want to distribute the heat because otherwise the temperature difference between the valve stem and valve itself gets very big so it can crack under thermal stress.
But it also has disadvantages. You apply it when the heat requires it. (so that's mostly on high specific output).
Another way to deal with the heat is to use inconel valves.
I think bmw has documents like this for every engine and every car. The nice thing is that for bmw most of these things can be found on the internet so you can learn a lot on your car from all that data. (I suspect most brands/types have these documents made for internal information distribution, but you find less of these floating on the internet).
For the E85 there is a 12 capitel pdf that adresses basically the whole car (including dme, driveline etc). I think that is part of an exam for bmw mechanics just like this document.
