I don't understand how these can work. How can the shade darken faster than the speed of light? Or is it that it darkens faster than enough energy can strike

I don't understand how these can work. How can the shade darken faster than the speed of light? Or is it that it darkens faster than enough energy can strike the eyeball -- you'd see a flash, but it would be super short and not long enough to totally cook your eye, perhaps only partially?