When our practice of Samatha arrives at calm, the mind will be clear and bright. The activity of mind will become less and less. The various mental impressions which arise will be fewer. When this happens great peace and happiness will arise, but we may attach to that happiness. We should contemplate that happiness as uncertain. We should also contemplate unhappiness as uncertain and impermanent. Well understand that all the various feelings are not lasting and not to be clung to. We see things in this way because theres wisdom. Well understand that things are this way according to their nature.
If we have this kind of understanding its like taking hold of one strand of a rope which makes up a knot. If we pull it in the right direction, the knot will loosen and begin to untangle. Itll no longer be so tight or so tense. This is similar to understanding that it doesnt always have to be this way. Before, we felt that things would always be the way they were and, in so doing, we pulled the knot tighter and tighter. This tightness is suffering. Living that way is very tense. So we loosen the knot a little and relax. Why do we loosen it? Because its tight! If we dont cling to it then we can loosen it. Its not a permanent condition that must always be that way.
We use the Teaching of Impermanence as our basis. We see that both happiness and unhappiness are not permanent. We see them as not dependable. There is absolutely nothing thats permanent. With this kind of understanding we gradually stop believing in the various moods and feelings which come up in the mind. Wrong understanding will decrease to the same degree that we stop believing in it. This is what is meant by undoing the knot. It continues to become looser. Attachment will be gradually unrooted.