A sudden blow: (line 1)
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.