Let me try, once
again, and as analytically yet simply as possible, explain why a "relic" knot still tied in the line of a just released loop is dangerous.
The moment the continuation of the returning eye leg ( the Tail End ) has been pulled out of the nipping structure tied on the continuation of the Standing End ( the Standing Part ), the loop "opens up", and it can not keep any object attached to the line at its other end,
stationary = at a constant distance from the anchor ( the bollard, the ring, the stake, etc ). Therefore, the object at the other end of the line, is now free to distance itself from the anchor, and
move, dragging the line along with it, just because the loop which has been "opened up", behaves like it is not encircling the anchor any more.
Now, two things may happen.
1. If the released loop was PET, by pulling the continuation of the returning eye leg out of the nipping structure, this nipping structure degenerates into the unknot, and the segment on which it was "knotted" becomes a straight line - which, as it is dragged by the moving object, does not run the danger to be caught on some obstacle it will meet in its path. A knot tied on a moving line can be stuck within a narrow passage, or be a factor a swirling line can become even more tangled and "knotted".
2. If the released loop was not PET, by pulling the continuation of the returning eye leg out of the nipping structure, this nipping structure remains "knotted", it is still forming a "relic" knot ( be it an overhand knot, a fig.8 knot, etc - but not un unknot ) - it is still forming a
stopper ! And guess why
stoppers are called like this !

This stopper, is not only annoying, because it has to be unknotted in a second stage. It may also become
dangerous, because, dragged by the object at the other end of it, the line of the released loop can slip around the rim of the bollard, the ring or the stake, and
move. A moving line, with a relic knot still tied within it, is something
dangerous, not because the knot which has already been partially untied runs any danger to become more unknotted, but because the remaining knot is still unknotted ! And in an environment where everything tends to move, this line with a now not-functional, useless nub on it, is asking for a trouble. We can not always manipulate lines and tie and untie knots, while they, or we, do not move ! THAT is what a sailor or a fisherman can understand, but Sweeney, for example

, can not ! The line may already be tensioned, or may be loose but become tensioned in the blink of the eye, because there are waves and gusts of wind out there, which push and pull the object / boat from all sides ! A loose line becomes tensioned, and vice versa, all the time, and at an instance. On such a line, a still knotted segment is potentially and probably dangerous, especially because, the moments just after an object free itself and is able to start moving away from the anchor, are the most critical. A previously static /
stationary system now becomes a dynamic /
moving system, and all the parameters which were determining the previous state now change - and they may change in an unpredictable, so potentially dangerous way.
One may argue that, immediately after he has released the 'first" knot, the one on the Tail End, he can very well untie the "second" knot, the one on the Standing Part, because both knots are tied on the same point, within the reach of the knot tyer s hands and fingers. However, this is exactly what a no-seaman, Sweeney, for example

( I use the same example, for no particular reason...

), can not understand : A loose line may become a tense line, and this may happen at an instance. Once the mechanism which immobilizes the whole three-part system ( anchor, line, object ), the loop, has been opened up, and it is not attached on the anchor any more, while it is still attached on the object which can now
move, the line is free to slip around anything it was attached on previously, AND THROUGH OUR FINGERS as well ! It is not only annoying to untie two "knotted" segments, it may also be impossible, during the short time frame and the under the unfavourable circumstances described above.
What am I doing ? I repeat the same things over and over again, when I
know that there is no ear nerve, or no ear drum, or no elastic medium between the ear and my mouth - and I am not postulating the existence of
brains, at the two far ends of this line !

The simplest advise is this : Tie and untie the infamous fake so-called "Zeppelin loop", many times. At the end, you will fall in
love with the PET loops !