How can those terrified vague fingers push The feathered glory from her loosening thighs?
This stanza consists of two rhetorical questions. The first question asks how Leda could possibly have prevented the rape.
The implication, of course, is that she couldn't have.
For one thing, she was too terrified, and for another, she was disoriented, and so her fingers were "vague" about what they would need to do to push away the swan.
(In case you were wondering, "feathered glory," is a not-so-thinly-veiled reference to the swan's penis. When you look closely, this poem is surprisingly graphic.)
"Vague" is a word worth noting. It makes you wonder if Yeats is implying that Leda is not dead-set against having sex with the swan/Zeus. "Vague" could mean indecisive in this sense: she doesn't know what she wants (not that she'd have a choice either way).
Modern day readers may well feel offended by the implication that a rape victim might not have been taken completely against her will.
We should say a word about how we might view this poem from an ethical perspective. For one thing, if this were a poem about a sexual assault involving two human beings, there's no way that any poet could get away with using this kind of language. That's because the poem is clearly intended to be sexy and erotic. But obviously, rape is not a turn-on; it's a serious crime. In order to fully understand Yeats's poem, we have to understand how Greek society and religion were different from our own.
Zeus was a god. Not just a god, but the chief god. In Greek mythology, the gods could do pretty much whatever they wanted: human standards of ethics didn't apply to them.
Many people find this poem or the myth that inspired it to be irresponsible. You could definitely take this stance.
Regardless of your position, when reading this poem, it is important to remember that Ancient Greeks had different values.
And how can body, laid in that white rush, But feel the strange heart beating where it lies?
The second rhetorical question of the second stanza asks how Leda could help feeling the swan's heart beating against her chest.
This question implies that the feeling of the heartbeat is exciting or mysterious.
We're still very close to Leda's perspective, and the swan's body is visible only as a "white rush" of fast-moving feathers.
The swan's heart is "strange" from two angles: it is both the heart of a god and the heart of an animal.
Yeats doesn't use any possessive pronoun to describe whose "body" it is.
You might expect, "And how can her body," but instead the line is, "And how can body".
It could be any body – yours, mine.
Yeats is warning us not to think that we would be any more successful at resisting Zeus than Leda. There's nothing weak or passive about Leda: she just doesn't stand a chance against a god.