"Why" is nice, but unless a teacher shows a student "how" it becomes an impasse. The diagram is nice, but I say "how?", IF the diagram is germane.
There is a lovely "pinky-nikkyo" that shows"how" but reveals little as to why it works -- the two points cannot be severed anymore than we fight on one leg.
Essentially, training this sort of thing progresses in this way (I'll get to the yin/yang thread topic presently, so bear with me). This is the first level:
1) identify the feel of the stable line, in any configuration
2) identify the feel of the departure from the stable line, in any dynamic
3) feed action into the shape of the collapsing line
This is in various modes what aikido calls kokyu tanden ho -- tui shou is very much concerned with same points. A slightly modified image from the pushed rope is that of metal cable -- you can push a little on it, in a very narrow range of positions, but outside of those it collapses -- but in a very typical coiling configuration depending on its internal stress -- point 3 is learning the coiling/uncoiling behavior, after departure from the line.
The next level learns to drive this interaction without an initial dynamic. Since we can feed into action that has tipped the stability cliff, action that provokes a departure will allow the driving of collapse, but only if we use yin-yang principles -- to provoke collapse up -- start down and then feed into the reaction. to provoke the left -- start right, etc. The recognition of the line together with learning its static manipulation creates inherently better structural stability.
This leads to understanding the two sides of the body working the opposed coiling and uncoiling tension/compression lines applying the "feed" action magnifying the collapse -- in phase and anti-phase relationship -- to apply left-right or up-down simultaneously, (tenchi
) creating an almost instantaneously buckle. The mastering of phase and anti-phase on both sides of the body leads to understanding the place in between them that they, in interaction, are provoking -- the 90 degree opposed stress or resonance interaction that creates the shear that is the cause of the coiling/uncoiling behavior. Once this is perceived, the applications can become much smaller yet more energetic.
As with a coiling cable, it is the initial stress state within the body itself that directs the "handedness" of the coils that result when collapse is begun. The amount or degree of that stress is irrelevant -- only its sign (left/right clockwise counter clockwise, etc.) and correct orientation of action relative to it matters to proper application.
The stress of the coiled cable can be reversed and with it the hand of the coils -- so can the stress within the body -- that coiling hand is all downhill and the opposite hand of coil is all uphill. Asagao
both illustrate the same opposed coiling stress principles, and the manner or shape of the continuous reversal.