The mind during Shikantaza
Look, there is nothing special about Shikantaza or any other type of meditation. If you ask me honestly, I say that meditation is for fun really. You don't do it to achieve anything. In fact the idea is to let everything be. And NOT achieve anything. No "Nirvana", No "Moksha", No "separation", No "clinging" or whatever labels we like to create. 

There is really no answer to this question apart from experience. You just sit and go on with whatever happens, your physical and mental sensations. Until they cease (or not). The moment you start thinking anything about it or whatever to do about it, the point is lost. 

What I mean is whatever you are thinking right now about the way to do it, or "am I doing it right?" or "what should I focus on?" or such like questions are mere thoughts and are invalid.