This is why when you stay long in a dark room, then move suddenly into light, your instinctive response is to squint. This automatic response is for the eye to reduce the amount of light entering the eye, to avoid eye damage. This gives time for the iris to respond and reduces the size of the pupil. When the iris relaxes and reduces the size of the pupil, then your eye acclimatizes to the bright light and the squinting stops.
When he is swinging down, all the kinetic energy is being used. Once he begins his ascent, immediately after the halfway point, potential energy builds up once again.
