A sudden blow: the great wings beating still Above the staggering girl, her thighs caressed
The swan comes in from above and knocks the girl off her feet.
This swan might be larger than average: his wings are huge.
He continues to flap his wings as he descends on Leda, who is "staggering" under its weight and trying to keep her balance. She totters back and forth, under siege from the bird.
The bird strokes her thighs.
The beginning of the poem takes us by surprise, as Leda is surprised by the ambush of the bird. In reality, few things would be more strange or shocking.
The first three words "A sudden blow," bring us close to Leda's perspective.
After being disoriented by the initial attack, she only catches scattered impressions of what happens next.
We see and feel the event as Leda would have. We don't have any distance from it, so we're left in the dark about important facts, which contributes to our general feeling of disorientation.
In addition, we don't know what form the "sudden blow" takes. Does the bird come in like a paratrooper and knock her over with its big feet, or does it whack her with its wings?
By the dark webs, her nape caught in his bill, He holds her helpless breast upon his breast.
Leda realizes what's caressing her thighs: the dark, rubbery webs of the swan's feet.
The swan begins to intertwine its graceful body with hers.
It grabs the back or "nape" of her neck with its bill.
In line 4, the swan is called "he" for the first time.
(Yeats expects that his readers are familiar with the myth of Leda and the swan and know that "he" means Zeus, the head honcho of the Greek gods. Zeus slept with hundreds of women and even some boys, too. In fact, Zeus did something similar to a boy named Ganymede. The god changed himself into an eagle and snatched up Ganymede to be his servant on Mt. Olympus.)
The Zeus-swan creature holds Leda's breast to its own feathery breast.
Leda is helpless and cannot stop the rape from taking place.