CPU-GPU game loop planification

I'm looking for some information regarding the Game Loop of Unity. I read http://docs.unity3d.com/Manual/ExecutionOrder.html before. I understood that the CPU was waiting for the entire frame to be rendered. However, a more realistic scenario would be the one exposed by Graham in this answer (http://answers.unity3d.com/questions/417909/cpu-wait-on-gpu-for-finish.html#answer-1016771):

" If the work the GPU has to do is big, then this way of working means the CPU is idle for a lot of the time. So, whilst the GPU is busy doing it's thing, control actually returns to Unity, and it then gets on working hard on the next frame. This is meant to allow the CPU and the GPU to be busy all the time."

Where can I read about this?

Regards.