In the Unity3D game engine, a common code sequence for getting remote data is this:

WWW www = new WWW("http://remote.com/data/location/with/texture.png");
yield return www;

What is the underlying mechanism here?

I know we use the yield mechanism in order to allow the next frame to be processed, while the download is being completed. But what is going on under the hood when we do the yield return www ?

What method is being called (if any, on the WWW class)? Is Unity using threads? Is the "upper" Unity layer getting hold of www instance and doing something?