In chapter 7 Joe has just fallen down the crevasse and at the beginning, he is bit confused as to what is happening. Joe uses contrast in the chapter to show his mixed feelings and thoughts, one moment he is thinking that he and Simon will die and the next he is thinking how amazing it is that he is alive, this is because he is very injured and delirious and his thoughts are all disjointed. Joe also has a lot of time with his thoughts and he comes up with contrast that is all accurate
The rope can be seen as a connection between Joe and Simon and when Joe is tugging on it hoping Simon is at the other end is representing how Joe still thinks Simon is alive and coming down to save him. When Joe realizes Simon is not at the other end it shows that Joe knows that he is on his own with no assistance from Simon.