Out of Core Rendering with Cycles

I’ve been googling about this (my google skills do suck), and not really seen much about it other than it exists.

Does it have to be enabled?
I’m presuming yes, and it requires the CPU and GPU to be selected. Is this correct?

What I was hoping is that I can have a scene render only using GPU, but if the GPU doesn’t have sufficient RAM, then drop to CPU or out of core, whatever works.