I believe the section of text that this question is asking about can be found in the closing paragraphs of chapter one. Ralph, Jack, and Simon have been exploring a little bit of the island. They are heading back to the rest of the boys, and they spot a piglet "caught in a curtain of creepers." The boys rush forward to get a closer look. Jack immediately pulls his knife and raises it into the air to strike and kill the pig. Then we are told that he pauses, and the pause was long enough for all three boys to understand what a serious thing that killing strike was going to be.
I think those lines show Jack's unwillingness to kill the pig. If he really wanted to kill the pig and was fully prepared to do it, there would not have been any further hesitation on his part. Pulling the knife and flourishing it in the air is teenage posturing—he's acting the brave part that he has been told he should be, but he doesn't really want to take on the potential implications of being a killer. That is why he hesitates.
He raised his arm in the air. There came a pause, a hiatus, the pig continued to scream and the creepers to jerk, and the blade continued to flash at the end of a bony arm. The pause was only long enough for them to understand what an enormity the downward stroke would be.