The way I understood it - or, rather, the way it was explained to me - is that you're supposed to do each in succession. You focus on the manifestation and let the universe know that this is what it is you want/need/expect, and then afterwards you let go and trust the universe to provide you with it. Apparently if you don't let go after putting it out there, it's the equivalent of not trusting the universe or repeatedly asking for the same thing from scratch, so it keeps working from that starting point and progress can't be made (kind of like running a race where you keep getting brought back to the starting line because they keep firing off the starting gun, if that makes sense).
Again, that's how it was explained to me.