Contrast is used in the text because of how Joe’s brain processed the situation. He was certain that death would claim him and Simon, to the point that he was at the point where he was on the edge of giving up and succumbing to the cold, accepting his death, with his only worry being that he wished for someone to find him or Simon and somehow know that they climbed the west face.
As Simon cuts the rope, in an effort to save himself, he hoped that Joe was close enough to the ground so that he wouldn’t fall and injure himself more or even to his death. The symbolic meaning of Simon cutting the rope in order to same himself severs his ties to Simon, When Joe realizes this when he tugs on the rope only to find that it’s been cut, he loses hope in Simon.
Joe begins lowering himself deeper into the crevasse Simply because he had no hope that he’d survive the situation, leading him to reach the conclusion to lower himself into the void.