A synecdoche is a figurative substitution of a part of something for the whole. For example, if one refers to a woman as "the blonde walking into the room," he means the entire woman but refers to her by only a part of her. From an analytical perspective, synecdoche often demonstrates directly or indirectly one's identity--a person's view of himself or someone else's view of him or her. This is true of the synecdoche in Lord of the Flies.
In Chapter 4 (where pages 63 and 64 fall) of the novel, Jack furthers his role as a predator by creating a mask for himself. As he makes it, his obsession blocks out those around him, and the mask swallows up his identity, liberating Jack from "shame and self-consciousness" (Golding 64). Jack uses the mask to frighten some of the other boys, and they begin to see only the mask and not Jack. In fact, the mask section closes with the statement,
"The mask compelled them" (64),
demonstrating that a part (the mask) has actually taken over the whole (Jack). It not only changes how Jack views himself, but it also begins to change the boys' view of Jack.