The Pring makes the comprehension of the entire text faster. So, if you just want to activate part of it you activate and then have met your purpose. However if you want the whole book activate the whole book. What you get/want out of a book should be stated in your purpose for photoreading the book. After you pred for a while the activation will come easier and you'll notice it feels more in context. The bonus is in the syntopic reading when you can cover several books on the same subject that it would normally take to cover the one book
Hope this helps.
Best of Luck,
Bruce
