The shock of the swan's attack on Leda represents the shock of sudden contact with another kind of reality. The heavy beats at the beginning of the poem fall like a lead weight. The "blow" explains Leda's disorientation throughout the poem.
The feathered glory from her loosening thighs? (line 6)
The word "glory" marks one of the first references to the swan's godlike nature. The poem makes clear that for Leda to resist would be futile. She is up against a force beyond her powers.
But feel the strange heart beating where it lies? (line 8)
Leda is caught between "higher" and "lower" natures (i.e., that of a god and that of an animal). The swan's heart is "strange" or foreign because it belongs to an animal and a divine being.
So mastered by the brute blood of the air,(line 12)
The word "brute" means beastly or animalistic – think of the word "brutal." But the "brute blood of the air" also makes the air sound like a supernatural being, a living force of nature with its own metaphorical blood. Even the air conspires against Leda in Zeus's favor.
Did she put on his knowledge with his power Before the indifferent beak could let her drop?(lines 13-14)
Even animals have power, and animals are usually considered to be "lower" creatures than humans. The speaker knows that Leda understands the swan's power – how could she not? – but he's curious about how much she understands of its supernatural being. Does she know that the swan is really Zeus? Did she take on or "put on" some of Zeus's omniscient knowledge? Or did it all happen too fast?