[Leigh Butler, Karan Mehra, Nathan Scott]
A lot of people were confused about the taint-cleansing at the end of WH. How exactly did that whole thing work?
Well, first it should be pointed out that a lot of the confusion stemmed from people trying to understand the cleansing in terms of physics. This ain't physics; it's magic, and therefore by its very nature not inclined to accommodate our idea of How Things Work.
That said, channeling is still a (mostly) internally consistent system of cause and effect, so we can use physics to construct analogies for what happened, in lieu of strictly literal explanations. We'll all just have to live with the fact that the analogy will not be perfect.
Rand's thoughts sum up the basis for the prevailing theory/analogy:
"The male and female halves of the True Source were alike and unalike, attracting and repelling, fighting against each other even as they worked together to drive the Wheel of Time. The taint on the male half had its opposite twin, too. The wound given him by Ishamael throbbed in time with the taint, while the other, from Fain's blade, beat counterpoint in time with the evil that had killed Aridhol" [WH: 35, With the Choedan Kal, 637].
Rand, using untainted saidar, formed a hollow tube, with one end touching the male half of the True Source and the other end touching Shadar Logoth. The weave did not form as he expected it to, but it worked as he intended.
He forced saidin through the pipe of saidar, pumping the tainted Power into Shadar Logoth. He did not, as some people thought, send all of saidin through SL, because that's impossible. What he seems to have done is pump saidin through the conduit until he felt the taint layered on top of it shift, and begin flowing into SL of its own accord, forming that huge black dome. Then he just had to hang on until all the taint went through into SL, and then collapse the conduit.
Basically it was a siphoning effect. Evan "Skwid" Langlinais explains: "The taint being the evil of The DO, and SL being an evil which was created to fight the DO's evil, the two attract one another like opposite magnetic polarities, and cancel one another out in the end. To get some of the taint into SL, Rand had to channel an immense amount of saidin, but this was still miniscule compared to the infinite quantity of saidin contained in the True Source. Once a sufficient quantity of taint was dumped into SL, SL consumed it at a rate greater than that of which Rand was dumping saidin into SL, and the taint was pulled along like it was being siphoned off the top."
This theory assumes two things:
The second item is not really in dispute, since to all appearances that's exactly how the taint behaved. However, there have been objections to the first, contending that there is no evidence that the taint and the SL evil would be actually attracted to each other.
Well, there are three reasons to think that the taint and the SL evil are attracted to each other. First, we saw examples, in TEOTW and elsewhere, that Mashadar did attack Trollocs and other Shadowspawn much more aggressively than it did ordinary humans. When Rand was being chased by Trollocs through SL in TEOTW, the tendrils of Mashadar only drifted toward Rand when he charged past, but when the Trollocs tried the same, the tendrils "swung uncertainly for a moment, then struck like vipers" [TEOTW: 20, Dust on the Wind, 251]. It stands to reason that the evil of SL would make the distinction, since creations of the DO are the original and (still) paramount target of Aridhol's hatred. It's not such a big leap to suggest that it would similarly be attracted to the taint, another creation of the DO.
Second, while in Caemlyn during TEOTW, Moiraine tells the gang that the SL dagger (which Mat was carrying at the time) would attract creatures of the Shadow to them. [TEOTW: 41, Old Friends and New Threats, 529]
And third, Rand's thoughts comparing the relationship of DO evil/SL evil to that of saidin/saidar explicitly indicate that they behave the same way. He specifically says "attracting and repelling". Since his plan appears to have worked just fine, there's no reason to think he was wrong.
Rand appears only briefly in COT, but that chapter makes clear that he is still experiencing dizziness and double vision whenever he wields saidin [COT: 24, A Strengthening Storm, 548]. So what's up with that? Assuming that the taint really has been cleansed, Rand's problems must come from some other source.
The dizziness problem didn't show up as a recurring affliction until TPOD. At the time it was thought that maybe it had something to do with the OP weirdness around Ebou Dar (see section 2.3.12), but the problem was both present before the Bowl was used, in ACOS, and has persisted since the phenomenon subsided (not to mention happening when Rand was nowhere near Ebou Dar, as well). So while the weirdness in Ebou Dar certainly didn't help Rand's condition, it could not have been the cause of it.
In [ACOS: 41, A Crown of Swords, 656-657], Rand is helped in his battle against Sammael in Shadar Logoth by a mysterious channeler (the Wanderer). When the two of them are threatened by Mashadar, both Rand and the Wanderer balefire the fog:
[Rand's] free hand rose, and balefire shot upward, a bar of liquid white fire slicing across the wave sinking toward them. Dimly he was aware of another bar of pale solid fire rising from the other man's hand that was not clasping his, a bar slashing the opposite way from his. The two touched.
Head ringing like a struck gong, Rand convulsed, saidin and the Void shattering. Everything was doubled in his eyes, the balconies, the chunks of stone lying about the floor. There seemed to be a pair of the other man overlapping one another, each clutching his head between two hands.
Since the Wanderer was Moridin (see section 1.2.3) and Rand didn't sense the use of saidin at all, Moridin's balefire had to have been created using the True Power. Since this is also the first time Rand experiences double vision while wielding saidin, it's logical to conclude that crossing the streams is Bad, and that this event is the source of Rand's dizziness problem.
It's not clear at this point why exactly the incident had this effect on Rand, but it's probably due either to an adverse reaction between the OP and TP, or to some bad paradoxical mojo caused by balefire trying to act upon other balefire.
Mike Edenfield expands on the latter idea: "The crossing of the balefire weaves is not the cause of Rand's problems, it's just the catalyst. The real cause was having an uber-strong weave balefired out of existence... This would basically undo the weave back in time up to several minutes, from historical balefire usage. It makes perfect sense, now that I think about it, for that effect to make saidin 'waver'... you are still channeling the balefire but suddenly you weren't really channeling it before now so how can you 'still' be channeling a weave that you never began etc etc. I can see how that'd seriously screw things up."
In KOD Rand has a vision of Moridin also becoming ill and being aware of his channeling when he weaves a gateway in Tear. [KOD 22: To Make an Anchor Weep] Aaron Cote suggests that perhaps Moridin is having problems channeling, and that's why he didn't show up to help the other Forsaken at Shadar Logoth at the end of WH.