Exercise 3 solution

When the line is loaded, the turns around the standing part are squeezed and grip, so the hitch holds. When you ease the tension and push the hitch, it slides, letting you tighten or loosen the line without untying.
It slips if the rope is slippery, if there are too few turns, or if the load comes off and the turns loosen.
Takeaway: more turns means more grip; ease the load before sliding it.