Turbo Compounding Explained?

Does anyone know exactly how turbo compounding works in a truck engine? It seems to be one of those technologies which appear and disappear without truck buyers actually working out how the thing works. This video doesn’t tell us anything except it will save US truck buyers fuel.

Over the years both Scania and Volvo have introduced and then un-introduced turbo compounding on their truck engines. The current DD 15 from Detroit does have an element of turbo compounding but, as to how it works, the answer is unclear.

What do we do when we don’t how it works? We go to YouTube for an explanation.

Here is a form of turbo compounding on a genset. The turbo’s energy is used to generate extra electricity, but it still isn’t clear if this is how it works on our truck engines:


Here is a video claiming to explain the whole concept, but I’m still not sure:


Here we have a bit more detail about the engine being introduced by Volvo into the US truck market next year. It seems to be converting the turbo energy into mechanical energy, but how?