... a symmetric spong ...This is the equivalent to what I described in the initial post
... looking quite different.
No, it is not "equivalent" ... It may
look like it, but it works very differently - so those two knots are as different as they might be !
In the modified spong knot, we are trying to utilize the same effect as at the rat tail stopper(1)(2)(3) : Each oblique round turn is squeezed upon the main line by a crossing oblique riding turn - and, at the next crossing, those two turns change roles : the riding turn passes "under", and becomes a round turn, that is now squeezed by the ex-round turn, that passes "over" and becomes a riding turn. In this way, the main line is deformed locally, at each and every node, and the gripping power of the multiple warps on it is greatly enhanced. This effect works as efficiently as it does in the rat tail stopper, only when the hitch is tied around a rope. A pole can not be deformed, so a similar hitch around a pole is not so effective ( ABoK#1755, 1756, KC hitch )
At the modified Prusik presented at this thread, the "coil tube" is continuous, so there is no local deformation of the main line at each point where the (two) oblique riding turns meet and squeeze the (multiple) round turns.
However, at the modified Prusik, the riding turns can be tensioned along their entire length much more than at the modified spong knot, or at the rat tail stopper - because they do not pass "under" the round turns at any point. This would probably be beneficial to the hitch s gripping power, for yet another reason : The oblique riding turns squeeze the adjacent round turns the one upon the other, and this can generate a very tight hitch - just as it does in the case of the multi-wraps Clove hitch mentioned previously.
1.
http://igkt.net/sm/index.php?topic=2849.02.
http://igkt.net/sm/index.php?topic=2191.msg16938#msg169383.
http://igkt.net/sm/index.php?topic=2191.msg16992#msg16992