RJ apparently used the scene of the Cleansing of saidin as a chance to "explain" the fundamental theory about the opposity between saidin and saidar and between the Shadow and Shadar Logoth. The main thing here is that the opposites of WoT are not the opposites of the both ends of a line, but the opposites of the diametral sides of a circle. As RJ is saying in an interview:

Quote:

"You don't think it's obvious? Err, let's see. You have.. You're using both repulsion and attration of opposites here. Repulsion of things that are opposite and repulsion of things that are the same. The Taint upon the... as versus the conduit, which is made of saidar through which the saidin passes. The saidin and saidar, as men and women, are in many ways opposite. It repels one another. It is safe to make this conduit of saidar between saidin and Shadar Logoth, because there can be no mixing. As the eh.. as saidin passes through, as the taint passes through, the saidar actually repels it, pushes it away from the ..., alright? Now, you have a taint on ... the eh source, the male half of the source, you have the taint on shadar logoth. They're not the same, yet they are. The Taint on Shadar Logoth did not come from the Dark One. The taint was created by humans, who believed that they must do whatever was necessary, anything that was necessary to defeat the Shadow. And because they would accept no limits to what they would do, to what could be done, to what needed to be done, they created their own destruction. Their evil is, or was, as great as that of the Dark One, but diametrically opposite. It is an evil created for the best of intentions, created for good intentions. So it is the opposite. So, this attraction created the conduit begins to pull the Taint from saidin to cipher it of. Remember, it's always been described it's not at mixed all through saidin , it is like a thin skin of rancidness, think of a thin skin of rancid oil floating on a pond, and if you get through it, you've got clean water, but you can't get through it without putting your hand in that oil. You're getting it on your hand... To attract one another because they are opposites, but because even being opposite, they have gone far enough around the circle, they act to destroy one another. You see, it's not opposites along a straight line. We're actually talking opposites along a circle. Continuing the motif of the Wheel of Time, if you will. So you've got two things that are both opposites and the same.That will both attract one another and negate one another."

Once, we have the statement, that the opposites are not opposites of a line, but opposites of a circle, it is a logical consequence that they are both opposite and the same. In a turning of an wheel the movement upwards and the movement downwards are both opposite and the same. Also, when you move in circle fast enough, both centrifigal and centripetal forces occur, so that any part of the wheel, while moved, is both attracted to the centre and repelled away from it.

Now there are methods of using exactly centrifugal and centripetal forces for dividing mixed stuff into clean parts. IMO Rand created a centrifuge to make the Cleansing. He noticed in his PoV just before the Cleansing, that the Taint and the SL-evil in his wounds both pulse but in opposite phase. Two things pulsing in opposite phase means, that when #1 is up, #2 is down, and when #1 is down, #2 is up. The same is true also about the opposite ends of a turning wheel. I.e. the taint and SL-evil are also the opposite directions of movement in the same circle.

So we have two circles: 1. saidin-saidar, 2. taint-SL. Rand moves saidin, and the saidar funnel is along the "surface" of saidin. Saidar is moving upwards, and saidin's natural movement is downwards (I use these distribution of "upwards" and "downwards", because if the stream of OP was a stream of a water in circular pipe, saidin is like rushing of the water down, and saidar is like moving it up because of the pressure): these both movements both affect the place on the surface of saidin, where the taint is, and so create extreme tension there, making the taint to move.

Quote from Winter's Hearth, ch.35:

"Drawing on saidin, fighting it, mastering it in the deadly dance he knew so well, he forced it into the flowery weave of saidar. And it flowed through saidin and saidar, like and unlike, could not mix. The flow of saidin squeezed in on itself, away from the surrounding saidar, and the saidar pushed it from all sides, compressing it further, making it flow faster. Pure saidin, pure except for the taint, touched Shadar Logoth."

So, saidin is sqeezed away from saidar and saidar presses saidin to move faster: the saidar is the flow under pressure and saidin is a flow that naturally runs away, so the saidar funnel with it's flower-like, rich of spirales form, is functioning like a propeller, like a fan. The movement of the flow of One Power can keep be movement only if there is movement in the both directions. This is the reason for saidin and saidar to cannot mix, despite of being two halves of the "one" Power. The movement of the one half requires the movement of the other half, and if one half moves more intensive, the other half will move laso more intensive ... in the opposite direction. Movement in the one direction causes also movement in the other direction.

Why this is important for moving the taint? Because the taint and the SL-evil form also a circle like the one of saidin and saidar, and the SL-evil is the opposite direction to the natural direction of movement of the taint (Shadar Logoth's idea was to oppose the Shadow with Shadow's means, i.e. it is the same force as the Shadow's effect, but in the opposite direction).

Quote:

"Awkwardly, forcing himself to work gently, to use the unfamiliar saidar's own immense strength to guide it as he wanted, he wove a conduit that touched the male half of the Source at one end and the distantly seen city at the other. The conduit had to be of untainted saidar. If this worked as he hoped, a tube of saidin might shatter when the taint began to leech out of it. He thought of it as a tube, at least, though it was not. The weave did not form at all as he expected it to. As if saidar had a mind of its own, the weave took on convolutions and spirals that made him think of a flower. There was nothing to see, no grand weaves sweeping down from the sky. The Source lay at the heart of creation. The Source was everywhere, even in Shadar Logoth. The conduit covered distance beyond his imagining, and had no length at all. It had to be a conduit, no matter its appearance. If it was not...."

Why it is so important to touch Shadar Logoth with saidin through a saidar tube? Because saidin is in the state of being pressed by saidar and the same kind of movement is to be induced in the Shadar logoth , so that the taint to be put into a tube of SL-evil. The movement in one direction causes the movement in the other direction: both for the saidar-saidin circle and the taint-mashadar circle. The movement in both direction forms a rotating circle. The moving taint (together with the moving saidin) must approach Shadar Logoth in polarized manner, in moving only in one direction to the geographical location of Shadar Logoth, for to force the SL-evil to move also in concentrated manner, one-directionally. This is the need of conduiting the saidin through untainted saidar: the saidin and therefore also taint will be focused and concentrated when apporaching Shadar Logoth.

The saidar funnel is in the same time on the surface of Saidin and in Shadar Logoth, so the movement, that is forced on the Taint, is the same movement that is forced on Shadar Logoth. And SL-evil begins also to move. But it moves in the opposite to the taint direction. So both together form a wheel. Or more precisely they form a whirlpool. The taint and the SL-evil force each other into an whirl, and the more taint comes in, the faster is the whirlpool, and the faster is the whirlpool, the closer it goes to it's centre. This whirlpool is the black dome. The combination of saidin movement and the saidar funnel cause overweighted centrifugal force, and that causes overweighted centripetal force in the Black Dome. The Black Dome sucks too much into it's centre, so, when it becomes too much, it collapses and implodes. But the Black Dome was not a bubble but whirlwind, i.e. it existed because of the movement, not because of mass of matter, so, when it collapsed it stopped to move, and when it stopped to move, it ceased to exist, and became a very little black hole. Thus the large hole that begins to form slowly in the place of Shadar Logoth, the hole that wasn't there when Cadsuane leaved the place, but that was miles deep, when Akkarin visited the place.

What happened with the Taint through the Cleansing process was experienced by Rand as shifting. And the more chaos and turbulent experience with the One Power, the more shifting:

Quote:

"Inside his head, Rand was screaming. He was sure that he was screaming, that Lews Therin was screaming, but he could not hear either voice in the roar. The foul ocean of the taint was flooding through him, howling with its speed. Tidal waves of vileness crashed over him. Raging gales of filth ripped at him. The only reason he knew that he still held the Power was the taint. Saidin could be shifting, flaring, about to kill him, and he would never know. That putrid flood overwhelmed everything else, and he hung on by his fingernails to keep from being swept away on it. The taint was moving. That was all that counted, now. He had to hold on!"

The taint because of the fast movement is engaged in the whirl with the SL-evil, and this attraction is stronger that the attachment of the Taint to the surface of saidin. So the taint moves from the saidin to Shadar Logoth. It would move back after the finishing of the process if the whirlwing haven't imploded. But it imploded and caused a black hole, and so was stuck in the location of Shadar Logoth maybe forever, and was not to return to the Source.