The poem begins in medias res (in the middle of the action), at the very moment that the god Zeus, in the form of a swan, has swooped down and hit Leda, a human woman and ancient Greek queen, with his enormous wings. As a result, Leda, described here as a "girl," stumbles, trying but failing to find her balance in the midst of this ambush.
Immediately, these lines establish the perspective of the speaker as very close to Leda's own. The first three words place the reader squarely within Leda's shock: she did not see the swan coming, and neither does the speaker or readers. This is further emphasized by the speaker's description of Leda as "staggering." She has not only been caught off guard but also violently struck, and is therefore reeling from this attack, barely able to remain on her feet.
Meanwhile, the description of Zeus's "great wings beating ... above" suggests that even in the form of a swan, the god is both larger and more powerful than Leda, and certainly larger and more powerful than an ordinary swan would be. Other than the image of the powerful swan beating its wings overhead, however, these lines reveal very little else about what's going on here. Again, this emphasizes how close the speaker is to Leda's own perspective. She's confused and overwhelmed by what is happening to her, so the speaker (and readers) are also disoriented.
These opening lines also introduce the poem's straightforward diction. The language is plainspoken, almost blunt, making clear what is happening here—a swan violently attacking a human woman. The meter here, on the other hand, is a bit unusual. The poem seems at times to be in iambic pentameter (meaning there are fie poetic feet, each with a da DUM stress pattern, per line)—the first indication that this poem is a sonnet, a traditional poetic form dating back to Petrarch and Shakespeare:
A sud- | den blow: | the great | wings beat- | ing still
Above | the stag- | gering girl,
However, the poem isn't consistent even from the start. The fourth foot of the first line ("wings beat-") is a spondee, a foot consisting of two stressed beats in a row; as a result, there are three stresses when describing Zeus's wings (great wings beat), which evokes the force and intensity of this action. The third foot of the second line, meanwhile, is an anapest (two unstressed beats followed by a stressed beat, "-gering girl")—another break in the meter that perhaps reflects Leda's own stumbling confusion. By breaking the rules right at the start, Yeats makes clear that the poem's form mirrors its provocative subject matter.