Resposta curta: Ninguém sabe.
Resposta longa:
Para saber como os elfos conseguiram sentir Sauron, precisamos entender como o anel Um funciona. O que segue é especulativo, pois não temos uma resposta definitiva.
Primeiro, vamos falar sobre Ósanwe. Ósanwe significa "comunicação do pensamento" em quenya. Tolkien escreveu um ensaio de oito páginas sobre o nome Ósanwe-kenta . Note que o narrador não é Tolkien, mas um loremaster élfico.
O que é Ósanwe?
Para ser muito simplista, é telepatia. Permite enviar pensamentos para outra mente e ler outras mentes.
Esta habilidade está disponível em teoria para todos os homens, elfos, maiar e Vala.
Na prática, os seres humanos são muito limitados:
Men have the same faculty as the Quendi, but it is in itself weaker, and is weaker in operation owing to the strength of the hröa, over which most men have small control by the will.
O mais poderoso dos homens, como Denethor, pode ler mentes à vontade. Enviar pensamentos é possível ao maior dos elfos:
Often after the hobbits were wrapped in sleep they [Celeborn, Galadriel, Gandalf, and Elrond] would sit together under the stars, recalling the ages that were gone and all their joys and labours in the world, or holding council, concerning the days to come. If any wanderer had chanced to pass, little would he have seen or heard, and it would have seemed to him only that he saw grey figures, carved in stone, memorials of forgotten things now lost in unpeopled lands. For they did not move or speak with mouth, looking from mind to mind; and only their shining eyes stirred and kindled as their thoughts went to and fro.
Toda a comunicação foi consensual, como qualquer mente poderia escolher permanecer fechada, exceto de Eru.
O esforço necessário para a comunicação pode ser reduzido por três fatores, afinidade, urgência, autoridade:
The Incarnates have by the nature of sáma the same faculties; but their perception is dimmed by the hröa, for their fëa is united to their hröa and its normal procedure is through the hröa, which is in itself part of Eä, without thought. The dimming is indeed double; for thought has to pass one mantle of hröa and penetrate another. For this reason in Incarnates transmission of thought requires strengthening to be effective. Strengthening can be by affinity, by urgency, or by authority. Affinity may be due to kinship; for this may increase the likeness of hröa to hröa, and so of the concerns and modes of thought of the indwelling fëar, kinship is also normally accompanied by love and sympathy. Affinity may come simply from love and friendship, which is likeness or affinity of fëa to fëa. Urgency is imparted by great need of the "sender" (as in joy, grief or fear); and if these things are in any degree shared by the "receiver" the thought is the clearer received. Authority may also lend force to the thought of one who has a duty towards another, or of any ruler who has a right to issue commands or to seek the truth for the good of others.
A perversão de Morgoth em Ósanwe
Então Morgoth (e depois Sauron) decidiu perverter Ósanwe e implantar seus próprios pensamentos na mente dos outros. No entanto, ainda é necessário consentimento :
In like manner, extortion of the secrets of a mind may seem to come from reading it by force in despite of its unwill, for the knowledge gained may at times appear to be as complete as any that could be obtained. Nonetheless it does not come from penetration of the barrier of unwill.
Então, como Morgoth fez isso?
He found that the open approach of a sáma of power and great force of will was felt by a lesser sáma as an immense pressure, accompanied by fear. To dominate by weight of power and fear was his delight; but in this case he found them unavailing: fear closed the door faster. Therefore he tried deceit and stealth. Here he was aided by the simplicity of those unaware of evil, or not yet accustomed to beware of it. And for that reason it was said above that the distinction of openness and active will to entertain was of great importance. For he would come by stealth to a mind open and unwary, hoping to learn some part of its thought before it closed, and still more to implant in it his own thought, to deceive it and win it to his friendship.
Aprimoramento tecnológico
Vários objetos foram criados para facilitar o uso de Ósanwe: o Palantiri, de Feanor.
O Um Anel
O que se segue é especulação.
Qual foi o poder do Anel:
“he could see and govern the very thoughts of those that wore them [the lesser rings].
Parece muito provável que o anel Um tenha tornado Ósanwe muito mais fácil entre os portadores do anel, dando a Sauron um backdoor para controlá-los.
No entanto, o consentimento é necessário. Sempre, pela vontade de Eru. O próprio Morgoth não sabia ler a mente de um hobbit se não estivesse disposto a fazê-lo.
Então, quando Sauron colocou o Anel, todos os portadores tiveram que consentir (ou pelo menos não recusá-lo ativamente). Os elfos tinham uma enorme vantagem, pois entendiam Ósanwe e já o haviam praticado. Então, eles sentiram a presença de Sauron em sua mente, entenderam o que era e o rejeitaram.