What makes the “beast” move in Chapter 6 of Lord of the Flies?
Golding answers this one quite clearly. The beast in this chapter is actually just a dead parachutist who falls from the skies. He is blown along until some rocks allow the...
strings of the parachute to tangle and festoon; and the figure sat, its helmeted head between its knees, held by a complication of lines. When the breeze blew the lines would strain taut and some accident of this pull lifted the head and chest upright so that the figure seemed to peer across the brow of the mountain. Then, each time the wind dropped, the lines would slacken and the figure bow forward again, sinking its head between its knees. So as the stars moved across the sky, the figure sat on the mountain-top and bowed and sank and bowed again.
So basically the answer is quite simple. Due to the wind blowing into the parachute and onto the strings, the figure is pulled up by its force - when the wind relents the figure falls again. Hence the "beast's" movement as SamnEric see it.
And when they see it, Golding is clear what is happening: describing "the plopping noise of fabric blown open".
Hope it helps - Merry Christmas!