A sudden blow: the great wings beating still Above the staggering girl, her thighs caressed (lines 1-2)
Leda's helplessness in the poem is linked to the "blow" she receives at the start of the poem. The assault has a vertical direction: she is knocked off her balance as the bird descends from above. She clearly lacks the ability to prevent what follows.
her nape caught in his bill, (line 3)
The swan gains control over Leda by grabbing the back of her neck with his bill. He wraps his neck around Leda. Although this act is an expression of raw power, critics also view it as an image of graceful and mysterious splendor to the reader's mind. The poem contains a tension between beauty and violence.
How can those terrified vague fingers push The feathered glory from her loosening thighs? (lines 5-6)
The second stanza contains the justification for Leda's lack of free will. She is too frightened ("terrified") and confused to take any course of action to stop what's happening (her fingers are "vague" about how to resist). The speaker's use of a rhetorical question suggests that the encounter is fated.