After the drilling of the Bore, the Dark One's access to the world was just through the Bore (Obvious). However, good old Lewis sealed the Bore. How is the Dark One touching the world again? Using a metaphor I have read on this site. Imagine that the patch is a sheet of metal, and the cuendillar disk are bolts. How can the Dark One toch the world but through the bolts? Of course, the bubbles of evil and sort "slipped through the cracks of His prison" as Moiraine put it (as the bolts lossen, you can slightly shift the patch. I think the way the Dark One tainted saidin is by routing the flow of saidin through the cuendillar disks. Maybe he originally did this so that the bolts would get worn away, and he could come out again, but by carrying the "taint" through the bolts, he could both wear the seals down every time someone channeled, and cause them to become as mad as Rand is.)