[link to original on tumblr]
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It corresponds to the phrase "in spring her voice she" in the lyrics. The "in spring" is G to F#, and "her voice she" is B C B, which - incidentally - is the same sort of trill that's in "Beechwood Park" and "Hung up on a Dream," among others. (In the mellotron parts at the beginning and end, I think that F# is sustained through that B C B part.)
I don't think I've run into too many half-steps in White's writing (I can't remember any at least); they're more common in Argent's writing. Argent usually links them in the middle (so that A to A# and A# to B combine into A to A# to B - a three-note chromatic phrase), which isn't the case here. It's interesting that while this is still the same sort of technique, it doesn't really resemble Argent's half-steps. They're half-steps, but they're not linked.