You should know the structure of the book after previewing and yanking out trigger words. Sometimes after that step alone I can pretty much guess where the important topics are in the book.
If you're anything like me you got all excited over the possibilities of Photoreading, you got it, figured out how much work it was and then felt disappointed? Welcome to Planet Earth. The good news is that if you're willing to do things RIGHT (not work hard) you can work away the dissappointment and become 10 times more excited than you were before only this time it won't go away.
You can't get more comprehension with PR than you had previously with your slow reading- at first. You can get 99% comprehension of what's important to YOU and 0% comp of what's not meaningful to you at this time though, so that balances out to 50% comprehension if you want to look at it that way. Statistically 15% or less of the words in a book have any relevance to the topic.
Most books contain WHAT and WHY. The WHAT could fill maybe 1 or 2 pages and the WHY could fill 1000 if the author felt like doing it. PR gets you to the WHAT very quickly and lets you control the degree of how much WHY you'll learn depending on how much you care about it. Speed and regular reading don't (doesn't?) discriminate between anything and you just start at the beginning and read everything no matter what it is. With PR that's the very last thing you do and most of the time you can stop before you get there.
HONESTLY if you were to slow read a book and then have to discuss it you probably wouldn't be able to cough up more than 5-10 main points along with the basic story line and that's after a couple of hours. Taking half the time and running thru the PR steps will give you more than that.
They say that comprehension is supposed to happen in layers but I think it happens more along the lines of that old tv gameshow "Concentration" or something like that. It's where there was a riddle or saying under pieces of a puzzle. When contestants got something right the peices of the puzzle were removed and more and more of the riddle was visible until someone figured out what the riddle was. PR puts the riddle there and activation removes the pieces covering it. But then again it's also like that Minesweeper game on your computer where if you hit one block all others touching it that are related are also removed. If your disappointed that just means that you haven't had this happen yet.