Normally when I used regular reading there was just some vague idea what I wanted to know and hoped the author had some information that might help me. Usually I didn't find it or didn't know that I found it. Simply because I didn't take a moment to establish my purpose.
When you have a purpose for reading there can be many levels.
At the text level the first question you ask is, 'Why do I want/need to read this or said another way. "Reading this, What's in it for me?" The answer can usually be stated, 'I want to satisfy my curiousity about...,'To know about... so that I can...,' or 'I don't know yet but it's required of me for school/work' So at this point you have your basic level purpose. An overall purpose for deciding whether or not you even need or want to read the text.
When you're curious to know something going in and reading it looking for the answer is straight forward and easy you made up your mind that the text is going to tell you something about ... and you expect to find the answer.
At this point you would probably preview the book to establish what it is about... how does it relate to the subject or studies you're interested in. What does this book discuss that you and can you decide if it's relevant to your purpose.
Say for example a gardener... picks up a gardening book, the book's purpose is to build a water feature. Now for the sake of example this gardener has to build the water feature for the course s/he... s/he ultimately wants to be a specilist in this area. At this stage of the course s/he is learning about pumping water for moving water displays. Unfortunately our student gardener knows nothing about plumbing, power and filters. So their purpose for that book and at this stage of the course is to fill that knowledge gap. The purpose has now been defined as to what s/he wants to get out of the book so that s/he can pass the course and ultimately become a landscape gardener speicalising in water features. It is an assigned textbook and at this point in the course the focus in the areas I mentioned.
Most readers would just read the book and hope it sticks and that they remember most of it when it comes up in class. Well our student gardener is a photoread... s/hes going to focus in the parts of the book that right now satisfy the immediate purpose, which is to fill that knowledge gap. So S/he will zero in on the parts of the book that deals with those issues. Asking questions to build their understanding. Going from getting a broad idea of pumping water for moving water, then getting into the details what pumps, how much water is required, how deep, when, where, why and what else. This is as much as s/he needs to know for the course. Now being a photoreader s/he discovers that there is a reading list and in it they notice a book wholely focused on this subject... so the go through that book too since it will ultimately be their specialty they don't mind knowing more than what is required for the class. In this case s/he is looking for specific for certain types of displays, again the when, where hows and whys.
The point of purpose is that it adds fuel to your reading/learning. Another way to say it is, 'After reading this, what benefit is it going to have in your life? What will you know, and how can you apply it?'
On a simplified level you know your purpose when you can answer someone (and in the case of books, answer the author),'What do you want to know? What do you need to know? So that you can...? With the focus of your global purpose (what you want to do in life) intermediate purpose (in your case pass this course so that I can meet my gobal purpose) and immediate purpose/s understand what I need to to build the foundation that supports my intermediate and global purpose.
So it always comes back to... "Why are you reading this. What are you looking for? How important is this to you. When do you need to know this by. How will you know you found the answer? Is this the right book? Is this the only book?
As for incubation time activating after Photoreading the forum post. They come into the category of short articles. No reall incubation time required. 
Alex