Shielding is the act of using the One Power to cut another off from the Power. Unlike severing, it is not permanent and the person will be able to channel again once the shield is gone and it may even be possible for them to break the shield themselves. The weave to do this is made of spirit (TDR, Ch. 54) and is very similar to the weave for severing, indeed, if the severing weave is dulled, it becomes the shielding one (TSR, Ch. 54). Shielding is easiest if the person does not hold the Power. If they do, then it is risky, unless you are much stronger than them, and even then, it is no certainty (WH, Ch. 11). Once shielded, however, a weaker person can hold the shield, though they can't be much weaker, unless they have a Talent in the area, such as Berowin (ACoS, Ch. 23). There is also apparently an alternate method of shielding, that used by Shaidar Haran, in which the One Power is not even there and there in nothing for the shielded person to work on or push against if they wish to escape; (LoC, Prologue; ACoS, Ch. 25; CoT, Ch. 21. A shielded person would however be able to use a Well (Robert Jordan Interview).
When men are taken for gentling, it is traditional to use a full circle of thirteen Aes Sedai. This seems to be mostly tradition, but Asmodean tells Rand that thirteen is a dangerous number and that he should be wary of such a circle, even if the women in it can barely channel on their own. Thirteen women shield Rand when they take him “thirteen to link and weave a shield no man could break” Once taken, fewer women are used. Both Rand and Logain were held with six (TGH, Ch. 24; LoC, Ch. 30; Ch. 51), and for Rand, not all of these maintained the shield, at times, some would tie their part off and leave (LoC, Ch. 55). Though six can hold a shield, six are unlikely to have been enough to capture either of those, as Guaire Amalasan and Yurian Stonebow were both able to overcome six Aes Sedai when they tried to capture them (LoC, Ch. 54).
Breaking a shield
Shields are stronger if they are maintained by the person who weaved them. If a shield is tied off, it is possible to use the knot as a point of weakness and unravel from that point, as long as it isn't too strong (TSR, Ch. 54; LoC, Ch. 55). The way they are tied off, can effect how easy this is. When Moghedien shields Liandrin, the knot is very intricate and she claims that Liandrin will neither be able to break it herself, or to find someone else who can (TFoH, Ch. 34). When Lanfear partially shields Asmodean, she claims the shield will fade over time, though he later states it has not done (TSR, Ch. 58). Once a shield has been tied off, or if there is is simply being held, the shield is broken by pushing at it with the mind, in which case it will stretch. If the person pushes enough, or is capable of doing so, it will shatter.
“When you learn enough to be raised to full sisterhood, no one woman will be able to do that to you. The stronger you become, the more Aes Sedai it will take to shield you against your will” (Siuan to Nynaeve, The Great Hunt, Chapter 18).
“It was as if she had run into another wall, a wall of clear glass; she could feel the True Source, but the wall stopped everything except the ache to be filled with the One Power” (Nynaeve, The Dragon Reborn, Chapter 54).
“Thirteen women who can barely channel could overpower most men, linked. The thirteen weakest women in the Tower could overpower you or any man and barely breathe hard” (Asmodean to Rand, The Fires of Heaven, Chapter 3).
"Thirteen to link and weave a shield no man could break, and two to bind him." (Rand, Lord of Chaos, Chapter 51).
Six points but one was hard now. Not soft; hard. And then a second. A third (...) A fourth knot became hard. He felt at the four hard points. (...) Cautiously, blindly, he felt at the four hard points. There was nothing there, any more than the shield itself was anything he could feel or see, but somehow he could feel around this nothingness, feel a shape to it. Like knots. (...). Slowly, ever so slowly, he fumbled into one of those gaps, squeezing through infinitesimal spaces between what seemed not to be there at all. (...). And suddenly he could feel the Source, like brushing it with a fingernail; the bare edge of-a fingernail. Saidin was still beyond him— the shield was still there—but he could feel hope welling in Lews Therin (...). It was as if he flexed what he had extended through the knot, flexed it as hard as he could. The knot resisted. It trembled. And then it burst. There were only five. The barrier thinned. He could feel it grow less. An invisible wall only five bricks thick now instead of six. The two Aes Sedai would have felt it, too, though they might not understand exactly what had happened, or how. Please, Light, not now. Not yet.
Quickly, almost frantically, he attacked the remaining knots in turn. A second went; the shield thinned. It was quicker now, quicker with each, as if he were learning the path through, though it was different each time. The third knot gone. And a third soft point appeared; maybe the Aes Sedai did not know what he was doing, but they would not simply sit while the shield grew less and less. Truly frantic, Rand hurled himself at the fourth knot. He had to unravel it before a fourth sister came into the shield; four might be able to hold it whatever he did. Almost weeping, he struggled through the complex windings, slipping between nothingness. Frenziedly, he flexed, bursting the knot. The shield remained, but held by only three now. If he could only move fast enough. When he reached for saidin, the invisible barrier was still there, but it no longer seemed stone or brick. It gave as he pressed, bending under his pressure, bending, bending. Suddenly it tore apart before him like rotted cloth. (Rand, Lord of Chaos, Chapter 55).
“She scrabbled at the shield. It might be hard as stone, but it felt more like glass, sleek and slippery. She could feel the source beyond it, almost see the Source, like light and warmth, just beyond the corner of vision(...) she felt her way across the smooth surface. It had an edge, like a circle, at once small enough to hold in her hands and large enough to cover the world, but when she attempted to slip round that edge, she found herself right back in the center of the slick hard circle again (.. There was one place where it felt...softer. She had never noticed that before. The soft point – a slight lump? - seemed no different in any other way from the rest and it was not much softer, but she hurled herself at it” (Nynaeve, Winter's Heart, Chapter 11).