[Hawk, Pam Korda]
When Rand has battled the male Forsaken, he has sometimes seen black threads, wires, or cords running off from them. This is seen when Rand battles Ishy in [TEOTW: 51, Against the Shadow, 637-640], and Asmodean in [TSR: 58, The Traps of Rhuidean, 671-675]. So what's the deal with these strings?
The thick black threads are the Forsaken's connection to the DO. The connection with the DO is what keeps the male Forsaken from falling prey to the madness from the taint on saidin (see Asmodean's statements to Rand in TSR after his black "thread" was severed). Rand's thread in TEOTW was silver and thinner because it was from that pure pool of saidin that was the Eye of the World.
We've never seen the threads on a female Forsaken, so it is possible that the threads are themselves the protection from the taint. Perhaps they act as a kind of filter on saidin, or a conduit through which the DO siphons off the taint when the Forsaken draws upon the Source.
It's also been suggested that the threads are a connection for channeling the True Power. This isn't very likely. The problems with this idea are:
Another suggestion is that the presence or absence of the threads could be used to identify male Forsaken. Unfortunately, this is not the case. The black cords are only seen/sensed under very special circumstances. Rand has only seen them in two places--T'A'R and the in-between space used for Skimming. Note that Rand has seen male Forsaken without seeing the cords: Aginor and Balthamel at the Eye, Be'lal in the Heart of the Stone, Moridin in Shadar Logoth, and Dashiva/Osan'gar on many occasions. Furthermore, he doesn't ALWAYS see the black threads when he's in T'A'R with a Forsaken--he never saw them on Rahvin. So, the black threads are not a reliable way to identify Forsaken under any circumstances, even those under which the threads have been seen in the past.