I am trying to understand the physics of a long rappel. Why do I need to add more friction towards the bottom of the rappel? Fore example, on a 100' rappel, I can use my device in low friction mode with no problems. On a 300' rappel, the same friction setting the last 100' will not be enough. I am assuming that it has to do with elongation on the upper portion of the rope but would like to confirm without taking a Newtonian physics course.