in some (most) grip hitches, it is the process of partial tightening which causes grip to start and once started, through a feedback mechanism it continues a positive feedback to further build the grip.
IMHO, this does not happen in "some", but only in a "few" gripping hitches - and, of course, it does not happen in "most" of them !
In particular, it does happen in the
rat-and-tail stopper, indeed, as at the
VT hitch - which
VT hitch is nothing but a shortened ( minimum length )
rat-tail-stopper, with a certain "closing". ( The two "cross gartered" helices of the
rat-tail-stopper can be joined together with
many different ways in their "lower" part, so the pull from the one, only, end can be transferred evenly to the other, too : It is paramount that the two crossing helices can work in tandem, and are evenly tensioned, otherwise such a hitch runs the danger to be distorted badly, with detrimental consequences in its gripping power ). However, as far as I know, it does NOT happen in any other of the many climbing hitches I am aware, because they are different ! ( Of course, it does happen also in the very similar hitches around poles shown by Ashley, but let us not mix apples with oranges here : the hitches around soft, compressible materials, as the ropes are, and the hitches around poles or wire rigging ). So, if the "
most" of the above sentence is changed to "
few", I agree. I, too, had in mind to point out this difference, but as it concerns a few only hitches, I decided not to - but it is good that Derek Smith made this comment, and it offers me the opportunity to explain what I mean.
Why it does happen ? Because, in those two hitches ( and possibly in a few, only, others, that I do not know...) the wraps are TWO crossed helices, and the angle of the coiling, relatively to the axis of the helix, is large. I do not know the
optimum angle ( I would be glad to learn it, because I guess there should be one optimum angle...), but let us say that this angle is / should be almost 45 degrees - while in "most" other gripping hitches, the helical coils are adjacent, so almost parallel to each other ( and the corresponding angle is almost 0 degrees...).
Why this large angle ? Because this angle makes the particular gripping mechanism of those hitches more effective : the friction between the two helices and the surface of the main line makes those helices to be elongated more, and the added length means a subtracted diameter, and the less diameter of each helical nipping/gripping tube means that the helices are squeezed around the main line and around each other more, offering even more friction, etc. Here the positive feedback mechanism mentioned by shadowjack works, indeed - but only
here !
In "most" climbing ripping hitches, the coils are meant to be and to remain parallel and adjacent to each other, and the "nipping/gripping" tube works as a whole : it is not as long and it is not elongated so much, so its diameter does not shrink so much, so the friction between the coils and the surface of the main line is not becoming greater GRADUALLY, as claimed.
I have to stress that those observations concern a really tightened main line : a free-hanging main line, like the main lines used in climbing, behaves differently, because it can be bent, locally, by the hitch. A tensioned anchor/mooring line of a ship can not be bent by the
rat-tail-stopper, of course !
( A double-line VT hitch is not less bulky, ugly and difficult to dress than a double-line rat-tail-stopper. I do NOT recommend those knots, they are NOT practical ! )