Jack plays emotion in order to manipulate the boys. Ralph and Piggy represent reason in the story, and try to reason with the boys by explaining the "truths", as they see them - the importance of the fire, the unlikeliness of the beast, etc.. However, Jack, as the symbol of emotion and instinct, is ultimately more effective in the end, as we see in the breakdown of civilization upon the island.
The two emotions that Jack focuses on the most are excitement and fear. The first he uses is excitement. He leads his choir boys into a hunting party because hunting is fun and exiciting. It is primal, and it makes the heart pump. The boys relish the adrenaline. Even Ralph is pulled in by the excitment of the hunt. After he hits the pig with the spear, he gets so caught up in the excitment that he boasts:
"I hit him all right. The spear stuck in. I wounded him." He sunned himself in their new respect and felt that hunting was good after all.
In the end, the boys turn the hunt into a play, and use Robert as the "pig." It is this that eventually turns Ralph off, because he notices in himself his own desire to get at Robert, who ends up crying and hurt. However, in all, the boys are enjoying their "games" and will remain loyal to jack for the excitment.
Back at camp, Jack uses the emotion of fear as a manipulation technique. When Ralph tries to emphasize the need for the fire, Jack plays up the horror of the "beast" to insist that there needs to be more focus on hunting and less on the fire. He scares the littleuns into hysterics, and Ralph loses control of the meeting, as Jack had planned.