I’m interested but can’t find out for myself because I don’t have a Pro license. I searched without success.
Sorry, I don’t understand the question: do you refer to soft or hard shadows? Do you refer to semitransparent shadow-receivers or shadow-casters? Do you want to know how it works in Unity (and why it doesn’t work without the Pro version) or are you interested in the algorithm? And why are multiple cameras relevant? (Oops, that were many more questions than the original question, I’d better stop here. 
)
I think maybe you aren’t aware that Unity licenses Umbra for occlusion culling. 
I would edit the title, but the forum won’t let me. 
OK, no, I didn’t know that, I thought you were talking about this http://en.wikipedia.org/wiki/Umbra