Why does Jack command the boys to dance and chant, and why is this an effective leadership tactic?
In chapter nine, Jack has fed the others meat and asserted his leadership over the group, challenging Ralph. The two get into a verbal conflict, with Ralph asserting he is, in fact, the chief. When it begins to rain, he ridicules Jack for not having built shelters. To divert the boys and get control of the situation, Jack orders them all to dance:
Jack leapt on to the sand. "Do our dance! Come on! Dance!"
Jack orders the dance so that he can gain ascendancy by tapping into the most primal and basest instincts of the mob. The dance, with its repeated chant of "Kill the beast! Cut his throat! Spill his blood!" moves the group away from reason and toward emotions and violence. It is an effective leadership tactic as groups are more often moved by emotion than logic.
By the end of the evening, the group has murdered Simon. The crime binds them together with a shared dark secret and solidifies Jack's control over the island, as Ralph recognizes in the next chapter.
*Question has been edited to a single question.
Jack commands the boys "Do our dance! Come on! Dance!" in chapter nine of Lord of the Flies after Ralph has questioned his authority in front of the other boys (150). Jack uses the dance and chanting as a diversionary tactic to draw attention away from Ralph's very serious and practical questions about the building storm and shelter. The dance serves as a unifying ritual, pulling all the boys into a frenzy of blood-lust, led by Jack, the most powerful hunter in the group. Jack's call to dance deflects the boys' focus from more practical matters; his tactic is both effective and manipulative. The chant unifies the boys into a common purpose, making it very easy for them to forget about their worries on the island.