I think we are missing the point on what Daro is doing. I believe he is trying to eliminate the 3 or 4 errors he will make over 400 or 500 birds. The logic might go like this: If the shot misses it is because the gun isn't pointing where you are looking when you are looking at the bird. The gun shoots where the bead is, providing you have set it up correctly. The solution then is to ensure that the eye-bead-bird relationship is preserved. So, teach yourself to preserve this relationship while looking at the bird and, when you miss, or your shots go off dead center, return to making sure this relationship is preserved. I believe Daro was trying to help us eliminate these errors early by cluing us in and getting us used to shooting this way.
Strictly from Trap shooting point of view. I think the "tunnel vision on the bird" theory is largely in agreement with most of what Daro is saying. Although preserving the eye-bead-bird relationship, is something I cannot agree with. The word "preserving" means that from the moment you call for the bird until you either break or lose the bird, the relationship doesn't change.
Again strictly from trap shooting point of view, that simply cannot be true. Here's a break down on what happens to your eye-bead-bird relationship when you shoot trap (observed over and over again in pros):
1. With the gun mounted you look ahead for the bird (or through the bead as Daro would say, so eye-bead relationship is bead is visible in the peripheral vision so (blur)
2. You call the bird and the motion of the bird gets registered by your eyes. Your eyes starts to trace down and focus on the bird, but your body have NOT started moving yet. Simple biology and physics here. Biology: when the eyes see the bird, it has to send information to the brain and the brain has to send order to your muscle to move, this takes time. Physics: things that are stationary wants to stay stationary until a force is applied to it, so your muscles have to accelerate the gun from 0km/hr to #km/hr to start moving, this takes time. Eye-bead-bird relationship is eyes see the bird clearly, but the bead is left behind, nowhere to be found (you described this as the bead disappears, which is fine)
3. Ideally your eyes continue track the bird clearly, and your body reacts to the moving bird, and the fixed block of head, gun, and upper torso moves as a whole unit, as if PULLED by the eyes, in the direction of the bird. At some point in this stage, you regain the blurry vision of the sight (which you mentioned as it reappearing), because the body is catching up to where your eye is looking
4. Still looking at the bird clearly, you see the blurry sight align with the bird, maybe with some lead, and you pull the trigger, while still swinging. At this point you either see a broken bird or still flying bird, but you should loose sight of your bead again, because you are still looking at the bird and the gun has "swept through" the bird.
As you can see, the eye-bead-bird relationship is a ever changing relationship. What is perserved is the eye-bird relationship, once you see the bird, do not look at anything else. The only other thing perserved, should be your head, gun, shoulder relationship as they should be moving as if they are one.
Assumptions: 1. You shoot both eyes open, 2. You are a swing through shooter.
Most people know that the head is supposed to be fixed on the gun, but forget that the shoulder needs to be fixed to the gun as well, the ability to do that, is dependent on the stance before calling the bird, but that's another topic I won't go into here.
But then again, maybe when Daro said perserve eye-bead-bird relationship he meant what I just said, what do I know

I've said it before and I will say it again, there is NO WRONG WAY, if you can do the wrong way CONSISTENTLY, then it can work for you.
Cheers.