By Chapter 5, the social order on the island, which never quite came together in the first place, has broken down. Jack, obsessed with hunting a wild boar, has siphoned support from Ralph's orderly leadership. There are competing interests on the island, but the primary contrast is that between work/order and play/chaos.
"If I blow the conch and they don't come back; then we've had it. We shan't keep the fire going. We'll be like animals. We'll never be rescued." (Golding, 102)
Having witnessed how Jack moves closer to violence and how his power over the boys increases because it is emotional and exhilarating rather than rational, Ralph becomes aware that he is losing control as a leader. In Chapter Five, he calls the boys to the platform in hopes of pulling them back to all that the meeting place has meant. He does not blow the conch but holds it above his head as though reminding them of its symbolic meaning.
The sun in his eyes reminded him how time was passing, so he took the conch down from the tree and examined the surface.... Ralph felt a kind of affectionate reverence for the conch.... He flourished the conch. (Ch. 5)
Ralph, who exemplifies order and respect for authority, has become aware that the dancing and mock hunt that Jack has engaged in with the others have produced a powerful emotional bonding effect. The boys' desire now is for immediate gratification, not civilized organization and construction or even rescue.
Ralph addresses the boys:
"Things are breaking up. I don't understand why. We began well; we were happy. And then.... We've got to talk about this fear and decide there's nothing in it." (Ch.5)
Ralph is worried that the boys will neglect the rescue fire and that they may become distracted from their jobs.
"Ralph took the conch from where it lay on the polished seat and held it to his lips; but then he hesitated and did not blow. He held the shell up instead and showed it to them and they understood."
This meeting occurs immediately after Samneric see the beast on the mountain. Samneric inform Ralph that they saw a living beast on the mountain, so Ralph's assumption is that something on this island is dangerous. He takes this into consideration before blowing the conch. If he were to sound the trumpet, the beast would know where to find the boys. His holding up the conch instead is a method of caution to avoid the beast.
This can be be further traced to the last meeting at the beginning of chapter five. The boys were just milling around the meeting area waiting for Ralph to sound the conch so their meeting can being. It's obvious that the conch itself isn't calling meetings anymore, so the significance of the sound is gone. Silently holding up the conch is still effective because the conch is still a symbol of power on the lsland although it's in this chapter that Jack starts to pull himself and other boys away from Ralph and the conch.