There is a thing called “Near-Field Myopia”. Basically, when your eyes have notning to focus on, they automatically focus at 3m (~ 10 ft). What he is sayi9ng to do is focus on something until you exclude the other items or images in the room. When this happens and all you see is the line or whatever else you are focused on. Then let your eyes become unfocused, the object will become blurry. Let your eyes natrually focus at 3m, and resist the instinct to focus.
At this point you can meditate, or do yourevocation. What this does is tells your mind to “see” whatever passes into your vision (part of the fight-or-flight instinct). When you are in this physically unfocused state, once you start to form images from your meditation, or a spirit comes as evoked, you are tricking your mind into thinking you are seeing it with your physical eyes, and will have less chance of disbelief, or being distracted by happenings in the physical temple, because you are paying attention to what is happening in your astral temple.