Frodo climbs to the summit of Amon Hen and sits upon the great stone chair. Still wearing the Ring, Frodo is able to see visions of what is transpiring in the lands he is gazing upon, mostly signs of war or coming war.
Frodo's eyes then drift toward Mordor. And then, for the first time, Sauron becomes aware of him:
And suddenly he felt the Eye. There was an eye in the Dark Tower that did not sleep. He knew that it had become aware of his gaze. A fierce eager will was there. It leaped toward him; almost like a finger he felt it, searching for him. Very soon it would nail him down, know just exactly where he was...he threw himself from the seat, crouching, covering his head with his grey hood.
He heard himself crying out: Never, never! Or was it: Verily I come, I come to you? He could not tell. Then as a flash from some other point of power there came to his mind another thought: Take it off! Take it off! Fool, take it off! Take off the Ring!
The two powers strove in him. For a moment, perfectly balanced between their piercing points, he writhed, tormented. Suddenly he was aware of himself again, Frodo, neither the Voice or the Eye: free to choose, and with one remaining instant in which to do so. He took the Ring off his finger.
Frodo's crisis is a fascinating moment where three powers collide, the Eye, the Voice, and Frodo's own will.
First off, we're reminded again of two of the big theological themes of the story. We're reminded of the irresistible power of evil when Frodo faces the Eye. And we also see the providential "something else at work" that comes to Frodo's rescue: "a flash from some other point of power." Over and over again, while God is not mentioned explicitly in the story, we've seen this "other point of power" at work in the story, meeting and thwarting the Eye at every turn.
And there's also something interesting at work here. The Voice doesn't force Frodo. Frodo still has to make a choice. He's still free.
In fact, I think you can make the argument that it's the arrival of the Voice that sets Frodo free. Without this intervention it seems certain Frodo would have succumbed to the Ring and the Eye. But when the Voice arrives, it doesn't act as the Eye would, it doesn't force or compel Frodo. Rather, the Voice sets Frodo free, creating space where Frodo can become aware of himself.
Where the Eye would dominate and compel obedience, destroying Frodo's will, the Voice frees Frodo will, and then places itself before Frodo as a choice he can make freely.
