What happens when Piggy holds up the conch and tries to talk?
Although Jack initially agrees to the rules about the conch and follows them at times, he tends to disrespect Piggy when Piggy holds the conch. In chapter 5, during the evening meeting, Piggy holds the conch and Jack starts defying Ralph's leadership and the rules. He argues with Ralph and finally shouts, "Bollocks to the rules!" and leads the boys down onto the beach in a "random scatter," preventing Piggy from saying what he intended to say.
Similarly, in chapter 6 when Piggy starts to speak, Jack breaks in and mocks him for always being scared. Then he says, "Conch! Conch! ... We don't need the conch anymore."
Despite Jack's disdain for the conch, Piggy still reveres its power as a symbol of authority. He insists they take it with them when they go to Castle Rock to demand Jack return Piggy's stolen glasses. As Piggy stands on the cliff going up to Jack's new camp, he is in line with the boulder that Jack has set up to keep the others out. Piggy begins by saying,"I got the conch," and then excoriates Jack and his tribe. During his speech, Roger, "with a sense of delirious abandonment, leaned all his weight on the lever." The boulder rolls toward Ralph and Piggy. Ralph sees it coming and jumps away, but Piggy, nearly blind, can't protect himself. The boulder strikes him and knocks him into the air. He falls forty feet onto the rocks and sea below. He dies from the fall. At the same time, "the conch exploded into a thousand white fragments and ceased to exist."
I assume you are referring to Chapter 11, when Ralph and Piggy are at Jack's camp in an attempt to get Piggy's glasses back. Jack and Ralph are fighting when Piggy raises up the conch to demand his glasses and that the fighting stop. Roger hits Piggy with a thrown rock, causing Piggy's fall and death.