I thought David Salo offered an interesting perspective of this in the news group, but I suppose that "Command" and "Word of Command" could be referring to the externalization of one's will.

Will, in Tolkien, is the root of power (although we see it most often in the corruptions of Melkor and Sauron). The One Ring offered control over other wills to its wielders (although not everyone could actually make use of such control).

Hence, Iluvatar through his Will creates the Ainur, and then he gives reality to their song by saying: Eä. "Let it be!"

The Ainur who enter Eä shape the universe and create worlds, and apparently they do so through their Will. An example of how they express their Will might be Yavanna's song where the Two Trees come to life: "...and Yavanna hallowed it, and she sat there long upon the green grass and sang a song of power, in which was set all her thought of things that grow in the earth."

Of course, Nienna assisted in bringing forth the Two Trees as well, but she too is "thinking": "But Nienna thought in silence, and watered the mound with tears."

Morgoth imposed his Will on Arda itself, infusing the world with his power, his being, his essence. And to Tolkien this was important, as when Sauron created the One Ring and the Elves created the other Rings of Power. The externalization of one's Will or power made it greater somehow. A Word of Command would be a small thing, perhaps, compared to a Ring or a Song of Power. But still it would represent the being's imposition of its Will on something beyond its own body.

Just a thought.

Michael

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Parma Endorion: Essays On Middle-earth, Revised Edition