This phrase occurs in the second rhetorical question of the second stanza. As with most of "Leda and the Swan", it is difficult to determine to whom this phrase refers. The full line is:
And how can body, laid in that white rush,
But feel the strange heart beating where it lies?
The body most likely refers to Leda's to further objectify her, where the "white rush" certainly refers to the feathers of the swan (rush generally referring to animal hair or grass). Line 8 is not so easily parsed -- must Leda listen to her own heart, or is it the swan who possesses a heart? Or is it meant to be both? If the heart belongs to Zeus (the most likely scenario), this gives the god-swan a strangely human quality of mortality; its "strangeness" only increases the incongruity of the image. If Leda is meant to understand this as a point of weakness in Zeus (and indeed, Zeus' apparent insatiability can certainly be called a weakness, a topic we will not discuss here), then she is also meant to understand that she still retains some amount of power and strength despite her compromising position.
The strange heart can also refer specifically to the swan's penis, as does the "feathered glory" of the previous line, where it lies between her thighs at the entrance to her vagina. This reading alters the question significantly: how could Leda in her present situation, thus truly and fully dominated, still refuse Zeus? The incredulity of the question, similar to that of the previous lines, poses a point of view that aligns more with Zeus and general maleness far more than Leda and femininity. However, the question also reveals that Leda is struggling against this conflict and does not comply of her own will, revealed by the shift in perspective from the first half of the octave.
This struggle against utter domination is a struggle familiar to all feminists, particularly during the First Wave in the UK and Ireland. In the 1920s, such domination of women by men was fairly commonplace and often unquestioned -- it was simply women's rightful place to be beneath men, literally and figuratively. Women were compliant mothers and quiet wives to their bread-winning husbands and needful children, and that was simply how it was meant to be. Here we see the power struggle at work in its most physical sense, but Yeats does not allow Leda to preserver, and this is because women have not been allowed to preserver. It is not a problem of strength or power but of thought and principle, and this is where Leda triumphs in the final lines of the poem.