i'm going to post here what i posted on your blog.
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while that does show a difference between the two, science fiction doesn't have to actually explain its stuff.
the main distinction between the two is that when writing science fiction, the author does actually figure out explanations for the wierd stuff, even if it isn't explained. in fantasy, it just happens. so while talking rabbits on a flying rocking horse could look the same in both, the main difference tends ot be that in science fiction the author tried to figure out why the rabbits are talking and the horse is flying, and thus figure out the limits and such of it. in fantasy it's just two rabbits and a horse.
the main reason you usually see the explanations given in science fiction is the fact that to a reader, unless the rabbits are described to be geneengineered, and the horse to be some anti-gravity vehicle, it will be assumed to be fantasy.
Orson Scott Card, author of many science fiction and fantasy novels, actually discusses an example of this in his book
how to write science fiction and fantasy. early in his career, he wrote a story set in a medieval type setting, with a charater who can talk to birds. he submitted it to a science fiction magazine, but it was regected because "they don't print fantasy." he was shocked, since in the background for his story it was set on a colony planet settled via a generation ship, and the main characters ability was a rare psychic trait. but because he hadn't put any mention of this in the story itself, it came off as a fairly typical magical fantasy setting.
so the main reason you see descriptions is so the reader will know wether the stuff in the story is magic or just sufficently advanced technology. it really doesn't matter too much to the story, aside from establishing the feel of the setting the writer is writing.