When you tell somebody to keep a high level of concentration, to concentrate 100 percent and not make any mistakes, that person becomes stupid and is liable to make more mistakes because he's concentrated on what he's doing. There's no gap. There's no room to open oneself, to room to relate with the back-and-forth play between the reference point of the object and the reference point of the subject. So the Buddha quite wisely advised that you put only tentative attention on your technique, not to make a big deal out of concentrating on the technique (this method is mentioned in the Samadhiraja Sutra.)
From "Continuing Your Confusion," in The Path Is the Goal: A Basic Handbook of Buddhist Meditation, page 19.