What the hell is "slack", this makes no sense.
The part of anything that hangs loose, having no strain upon it.
Slack doesn't describe it 100% accuratenely, but it's what came to mind when I wrote the script. Increasing the slack value makes the rope hang down more. Technically, it's the number of units the lowest point of the rope hangs down from the highest part. If you put a negative value in, you would get a gravity defying rope.