In reply to winhill:
I must be explaining this badly because it's a couple of a fairly simple concepts. I'll try again.
> I can't see how it helps with your peripheral vision
If you're looking up at your belayer without the glasses, your peripheral vision mainly covers the rest of the crag above you (i.e. the other lines), because your head is facing the sky.
If you're looking up at your belayer with the glasses, your peripheral mainly covers the base of the crag (the rope, people/dogs wandering around you, etc.) because your head is pointed forwards.
Likewise if you glance down at the rope bag without the glasses, your peripheral vision covers the base of the crag - you cannot see your climber.
If you glance down at the rope bag with the glasses on, your peripheral vision will still contain the glasses and therefore the climber - any sudden movement above from a falling climber will attract your attention much quicker than tension on the rope will.
Furthermore, to make that glance down to your rope bag, you don't need to move your head - moving your eyes is much faster than moving your head meaning there's less time between your focus leaving the climber and it returning to them.
> But again, I can't see how this makes you more attentive, no one I've spoken to ever said that and it wasn't my experience and I can't see how that would work and I don't think CU would add that to their marketing.
Go down any wall, or any crag, and you'll see belayers stretching their necks whilst their climber is on lead.
If your neck and shoulders are in pain, with the best of intentions, it's very difficult to keep squinting up at your climber for every second he or she is on lead on a long day at the crag.
If you're standing in a neutral position, the only reason not to keep looking at your climber would be boredom.