Suppose you knew that before!
My allegation is if you believe that you know sth or think you will learn it certainly your concentration will increase and you'll sink in the stuff.
As instances I can address my personal experiences:
1)remembering a name-for example a teachers name- or sth like this, perhaps you've encountered that in your life.and you would confirm my allegation.it is really strange you sink completely and feel on edge!
2)prestudy : I myself had just experience that while doing that for structural analysis and fluid mechanics courses.it worked and as a result I haven't read a single word from my structural analysis pamphlet yet and confidently and happily hand in my assignments!
3)learning language : this case is virtually similar to the last case.
if you learn a lot of words-no matter to master them just 30 -40 percent of mastery is enough- you will get that sense and you will concentrate more on words and language structure.