the great wings beating still Above the staggering girl (lines 1-2)
Zeus transforms himself into a swan in order to rape Leda, but he's not any ordinary swan. The "great wings" of the swan represent Zeus's superhuman power. When a mythical person or god changes form to become something else, the term is metamorphosis.
And how can body, laid in that white rush, But feel the strange heart beating where it lies? (lines 7-8)
Zeus's transformation is so complete that Leda cannot recognize him: his heart feels "strange" or alien.
A shudder in the loins engenders there The broken wall, the burning roof and tower And Agamemnon dead. (lines 9-11)
The crucial point of historical transformation occurs when Zeus impregnates Leda with Helen of Troy. Yeats wants us to pay attention not just to the sexual act as a turning point, but literally to the exact moment of conception, represented by a "shudder" in the swan's loins. He traces a direct historical line from that moment to the destruction of the great ancient city of Troy.
Did she put on his knowledge with his power Before the indifferent beak could let her drop?(lines 13-14)
Did Leda transform herself by channeling Zeus's knowledge? If Leda were truly to be like a god, she would have to become immortal, but that doesn't happen. However, gaining the knowledge or insight of a god is another important kind of power. The ending of the poem raises many questions, such as whether Zeus himself knows that having sex with Leda will lead to the Trojan War. These questions are left unanswered.