Originally Posted by Reply
Is there a non-highly-technical explanation for this?
Tying a knot in the rope results in kinks and bends, which focus the forces on the rope onto a smaller area. The tighter the turns made by the rope in the knot, the weaker the knot is compared to unbent rope. Suppose a rope rated to 100 lbs is made up of ten fibers, each of which can support 10 lbs without breaking. If you lay the rope over the 90 degree edge of a table and hang 100 lbs from it, the fibers on the inside of the bend and the outside of the bend will be differently loaded, to the point that some individual fibers will exceed their 10 lb maximum. Once those fibers part, the load is spread to the remaining fibers, which fail in turn.