Everything has already been said.
All I can add is that it's simply a natural process as with any other job or activity you do. You start slowly, having no clue what you are doing, you then improve slowly, find something that works, then you naturally create reflexes, which speed up your process and eventually multi task. And I believe this is just a natural progress in poker. When you start playing poker, you don't really recognise anything but the cards, but slowly your brain starts to regonise patterns, which give you reads and information. Once you have the reflexes, you just apply them to more and more tables, because your brain already knows what to expect, of course with some tough decisions here and there, but not as tough as all decisions used to be in the beginning. When you move up the stakes, you start again from the bottom, with some idea but still won't know what to expect until you see it happen 1000 times.