After the meeting held in Chapter 5 in which the fear of the beast is discussed without any resolution, the boys have trouble sleeping in their insecurity and wishing for parents. While they sleep there is a bright explosion, then darkness.
a figure dropping swiftly beneath a parachute, a figure that hung with dangling limbs. The changing winds of various altitudes took the figure where they would.
With this wind, the figure "bowed and sank and bowed again." Away from the others as sentries, Sam and Eric rekindle the fire which they let go out at night as they both slept; in so doing, they hear "the plopping noise of fabric blown open." With the early dawn and shadows from the fire and the talk of the previous night, they become terrified and flee.
Rushing to the others, they tell of "the beast." It is "furry" and has something like wings flapping. Their imaginations fueled by the fright of the previous night, they imagine that the beast has nearly touched them. Even Piggy becomes frightened by the mass hysteria of the group.
This fright of the twins and the others is an example of the all too human herd condition in which people instinctively follow one another's movements or thoughts.