What are the decisions the poet has made about form? How do these decisions affect meaning?

Margaret Atwood, in her poem "Siren Song," chose to write in free verse, using short stanzas of three lines each. The most interesting effect of this is that each of the nine stanzas does not so much contain an idea as present half of an idea, with the enjambment between the stanzas serving to keep the reader in suspense and encourage us to pay attention to the content of the poem. The punctuation at the end of the first stanza, for example, seems to echo the conversational tone of the poem in that it represents an indrawn breath: a colon introducing a new idea, a longed-for answer, which we must continue to the next stanza to hear.
The isolated stanzas create the impression that the speaker is feeding her ideas in small pieces, the poem itself a lure and a siren song. Consider this stanza:

I will tell the secret to you,to you, only to you.Come closer. This song

These lines seem to draw the reader in, as if the speaker is beckoning ("come closer"), and the abrupt break in the sentence at the end of the final line has the reader leaning closer still, hurrying onto the next stanza. The final stanza, which reveals the secret of the "boring song" which "works every time," pulls the reader up short; finally we understand that the siren has been working her magic on us after all, demonstrating, rather than explaining, the "secret" of how she uses her "cry for help" to make each listener feel "unique."